wit_parser/resolve/

mod.rs

use alloc::borrow::ToOwned;
use alloc::collections::BTreeMap;
use alloc::string::{String, ToString};
use alloc::vec::Vec;
use alloc::{format, vec};
use core::cmp::Ordering;
use core::mem;

use crate::resolve::error::{ResolveError, ResolveErrorKind};
use crate::*;
use anyhow::{Context, anyhow, bail};
#[cfg(not(feature = "std"))]
use hashbrown::hash_map::Entry;
use id_arena::{Arena, Id};
use semver::Version;
#[cfg(feature = "serde")]
use serde_derive::Serialize;
#[cfg(feature = "std")]
use std::collections::hash_map::Entry;

use crate::ast::lex::Span;
use crate::ast::{ParsedUsePath, parse_use_path};
#[cfg(feature = "serde")]
use crate::serde_::{serialize_arena, serialize_id_map};
use crate::{
    AstItem, Docs, Function, FunctionKind, Handle, IncludeName, Interface, InterfaceId,
    LiftLowerAbi, ManglingAndAbi, PackageName, SourceMap, Stability, Type, TypeDef, TypeDefKind,
    TypeId, TypeIdVisitor, TypeOwner, UnresolvedPackage, UnresolvedPackageGroup, World, WorldId,
    WorldItem, WorldKey,
};

pub use clone::CloneMaps;

mod clone;
pub mod error;

#[cfg(feature = "std")]
mod fs;
#[cfg(feature = "std")]
pub use fs::PackageSourceMap;

/// Representation of a fully resolved set of WIT packages.
///
/// This structure contains a graph of WIT packages and all of their contents
/// merged together into the contained arenas. All items are sorted
/// topologically and everything here is fully resolved, so with a `Resolve` no
/// name lookups are necessary and instead everything is index-based.
///
/// Working with a WIT package requires inserting it into a `Resolve` to ensure
/// that all of its dependencies are satisfied. This will give the full picture
/// of that package's types and such.
///
/// Each item in a `Resolve` has a parent link to trace it back to the original
/// package as necessary.
#[derive(Default, Clone, Debug)]
#[cfg_attr(feature = "serde", derive(Serialize))]
pub struct Resolve {
    /// All known worlds within this `Resolve`.
    ///
    /// Each world points at a `PackageId` which is stored below. No ordering is
    /// guaranteed between this list of worlds.
    #[cfg_attr(feature = "serde", serde(serialize_with = "serialize_arena"))]
    pub worlds: Arena<World>,

    /// All known interfaces within this `Resolve`.
    ///
    /// Each interface points at a `PackageId` which is stored below. No
    /// ordering is guaranteed between this list of interfaces.
    #[cfg_attr(feature = "serde", serde(serialize_with = "serialize_arena"))]
    pub interfaces: Arena<Interface>,

    /// All known types within this `Resolve`.
    ///
    /// Types are topologically sorted such that any type referenced from one
    /// type is guaranteed to be defined previously. Otherwise though these are
    /// not sorted by interface for example.
    #[cfg_attr(feature = "serde", serde(serialize_with = "serialize_arena"))]
    pub types: Arena<TypeDef>,

    /// All known packages within this `Resolve`.
    ///
    /// This list of packages is not sorted. Sorted packages can be queried
    /// through [`Resolve::topological_packages`].
    #[cfg_attr(feature = "serde", serde(serialize_with = "serialize_arena"))]
    pub packages: Arena<Package>,

    /// A map of package names to the ID of the package with that name.
    #[cfg_attr(feature = "serde", serde(skip))]
    pub package_names: IndexMap<PackageName, PackageId>,

    /// Activated features for this [`Resolve`].
    ///
    /// This set of features is empty by default. This is consulted for
    /// `@unstable` annotations in loaded WIT documents. Any items with
    /// `@unstable` are filtered out unless their feature is present within this
    /// set.
    #[cfg_attr(feature = "serde", serde(skip))]
    pub features: IndexSet<String>,

    /// Activate all features for this [`Resolve`].
    #[cfg_attr(feature = "serde", serde(skip))]
    pub all_features: bool,

    /// Source map for converting spans to file locations.
    #[cfg_attr(feature = "serde", serde(skip))]
    pub source_map: SourceMap,
}

/// A WIT package within a `Resolve`.
///
/// A package is a collection of interfaces and worlds. Packages additionally
/// have a unique identifier that affects generated components and uniquely
/// identifiers this particular package.
#[derive(Clone, Debug)]
#[cfg_attr(feature = "serde", derive(Serialize))]
pub struct Package {
    /// A unique name corresponding to this package.
    pub name: PackageName,

    /// Documentation associated with this package.
    #[cfg_attr(feature = "serde", serde(skip_serializing_if = "Docs::is_empty"))]
    pub docs: Docs,

    /// All interfaces contained in this packaged, keyed by the interface's
    /// name.
    #[cfg_attr(feature = "serde", serde(serialize_with = "serialize_id_map"))]
    pub interfaces: IndexMap<String, InterfaceId>,

    /// All worlds contained in this package, keyed by the world's name.
    #[cfg_attr(feature = "serde", serde(serialize_with = "serialize_id_map"))]
    pub worlds: IndexMap<String, WorldId>,
}

pub type PackageId = Id<Package>;

/// Source name mappings for resolved packages (no_std compatible).
#[derive(Clone, Debug)]
pub struct PackageSources {
    sources: Vec<Vec<String>>,
    package_id_to_source_map_idx: BTreeMap<PackageId, usize>,
}

impl PackageSources {
    pub fn from_single_source(package_id: PackageId, source: &str) -> Self {
        Self {
            sources: vec![vec![source.to_owned()]],
            package_id_to_source_map_idx: BTreeMap::from([(package_id, 0)]),
        }
    }

    pub fn from_source_maps(
        source_maps: Vec<SourceMap>,
        package_id_to_source_map_idx: BTreeMap<PackageId, usize>,
    ) -> PackageSources {
        for (package_id, idx) in &package_id_to_source_map_idx {
            if *idx >= source_maps.len() {
                panic!(
                    "Invalid source map index: {}, package id: {:?}, source maps size: {}",
                    idx,
                    package_id,
                    source_maps.len()
                )
            }
        }

        Self {
            sources: source_maps
                .into_iter()
                .map(|source_map| source_map.source_names().map(|s| s.to_owned()).collect())
                .collect(),
            package_id_to_source_map_idx,
        }
    }

    /// All unique source names.
    pub fn source_names(&self) -> impl Iterator<Item = &str> {
        self.sources
            .iter()
            .flatten()
            .map(|s| s.as_str())
            .collect::<IndexSet<&str>>()
            .into_iter()
    }

    /// Source names for a specific package.
    pub fn package_source_names(&self, id: PackageId) -> Option<impl Iterator<Item = &str>> {
        self.package_id_to_source_map_idx
            .get(&id)
            .map(|&idx| self.sources[idx].iter().map(|s| s.as_str()))
    }
}

/// Visitor helper for performing topological sort on a group of packages.
fn visit<'a>(
    pkg: &'a UnresolvedPackage,
    pkg_details_map: &'a BTreeMap<PackageName, (UnresolvedPackage, usize)>,
    order: &mut IndexSet<PackageName>,
    visiting: &mut HashSet<&'a PackageName>,
    source_map_offsets: &[u32],
) -> ResolveResult<()> {
    if order.contains(&pkg.name) {
        return Ok(());
    }

    let (_, source_map_index) = pkg_details_map
        .get(&pkg.name)
        .expect("No pkg_details found for package when doing topological sort");
    let offset = source_map_offsets[*source_map_index];
    for (i, (dep, _)) in pkg.foreign_deps.iter().enumerate() {
        let mut span = pkg.foreign_dep_spans[i];
        span.adjust(offset);
        if !visiting.insert(dep) {
            return Err(ResolveError::from(ResolveErrorKind::PackageCycle {
                package: dep.clone(),
                span,
            }));
        }
        if let Some((dep_pkg, _)) = pkg_details_map.get(dep) {
            visit(
                dep_pkg,
                pkg_details_map,
                order,
                visiting,
                source_map_offsets,
            )?;
        }
        assert!(visiting.remove(dep));
    }
    assert!(order.insert(pkg.name.clone()));
    Ok(())
}

impl Resolve {
    /// Creates a new [`Resolve`] with no packages/items inside of it.
    pub fn new() -> Resolve {
        Resolve::default()
    }

    /// Merge `main` and `deps` into this [`Resolve`], topologically sorting
    /// them internally. Returns the [`PackageId`] of `main` and a
    /// [`PackageSources`] covering all groups.
    fn sort_unresolved_packages(
        &mut self,
        main: UnresolvedPackageGroup,
        deps: Vec<UnresolvedPackageGroup>,
    ) -> ResolveResult<(PackageId, PackageSources)> {
        let mut source_maps: Vec<SourceMap> = Vec::new();
        let mut all_packages: Vec<(UnresolvedPackage, usize)> = Vec::new();

        let mut collect = |group: UnresolvedPackageGroup| {
            let UnresolvedPackageGroup {
                main,
                nested,
                source_map,
            } = group;
            let i = source_maps.len();
            source_maps.push(source_map);
            for pkg in nested.into_iter().chain([main]) {
                all_packages.push((pkg, i));
            }
        };

        let main_name = main.main.name.clone();
        collect(main);
        for dep in deps {
            collect(dep);
        }

        // Merge all source maps into resolve.source_map upfront so that every
        // span produced during toposort and duplicate detection is valid in
        // resolve.source_map and can be located by rewrite_error below.
        // Each group has exactly one source map, so a Vec of offsets suffices.
        let source_map_offsets: Vec<u32> = source_maps
            .iter()
            .map(|sm| self.push_source_map(sm.clone()))
            .collect();

        let mut pkg_details_map: BTreeMap<PackageName, (UnresolvedPackage, usize)> =
            BTreeMap::new();
        for (pkg, source_map_index) in all_packages {
            let name = pkg.name.clone();
            let my_span = pkg.package_name_span;
            let offset = source_map_offsets[source_map_index];
            if let Some((prev_pkg, prev_source_map_index)) =
                pkg_details_map.insert(name.clone(), (pkg, source_map_index))
            {
                let prev_offset = source_map_offsets[prev_source_map_index];
                let mut span1 = my_span;
                span1.adjust(offset);
                let mut span2 = prev_pkg.package_name_span;
                span2.adjust(prev_offset);
                return Err(ResolveError::from(ResolveErrorKind::DuplicatePackage {
                    name,
                    span1,
                    span2,
                }));
            }
        }

        // Perform a simple topological sort which will bail out on cycles
        // and otherwise determine the order that packages must be added to
        // this `Resolve`.
        let mut order = IndexSet::default();
        {
            let mut visiting = HashSet::new();
            for (pkg, _) in pkg_details_map.values() {
                visit(
                    pkg,
                    &pkg_details_map,
                    &mut order,
                    &mut visiting,
                    &source_map_offsets,
                )?;
            }
        }

        let mut package_id_to_source_map_idx = BTreeMap::new();
        let mut main_pkg_id = None;
        for name in order {
            let (pkg, source_map_index) = pkg_details_map.remove(&name).unwrap();
            let span_offset = source_map_offsets[source_map_index];
            let is_main = pkg.name == main_name;
            let id = self.push(pkg, span_offset)?;
            if is_main {
                assert!(main_pkg_id.is_none());
                main_pkg_id = Some(id);
            }
            package_id_to_source_map_idx.insert(id, source_map_index);
        }

        Ok((
            main_pkg_id.unwrap(),
            PackageSources::from_source_maps(source_maps, package_id_to_source_map_idx),
        ))
    }

    /// Appends a source map to this [`Resolve`]'s internal source map.
    ///
    /// Returns the byte offset that should be passed to [`Resolve::push`] for
    /// packages parsed from this source map. This offset ensures that spans
    /// in the resolved package point to the correct location in the combined
    /// source map.
    pub fn push_source_map(&mut self, source_map: SourceMap) -> u32 {
        self.source_map.append(source_map)
    }

    /// Appends a new [`UnresolvedPackage`] to this [`Resolve`], creating a
    /// fully resolved package with no dangling references.
    ///
    /// All the dependencies of `unresolved` must already have been loaded
    /// within this `Resolve` via previous calls to `push` or other methods such
    /// as [`Resolve::push_path`].
    ///
    /// The `span_offset` should be the value returned by
    /// [`Resolve::push_source_map`] if the source map was appended to this
    /// resolve, or `0` if this is a standalone package.
    ///
    /// Any dependency resolution error or otherwise world-elaboration error
    /// will be returned here, if successful a package identifier is returned
    /// which corresponds to the package that was just inserted.
    pub fn push(
        &mut self,
        mut unresolved: UnresolvedPackage,
        span_offset: u32,
    ) -> ResolveResult<PackageId> {
        unresolved.adjust_spans(span_offset);
        let ret = Remap::default().append(self, unresolved);
        if ret.is_ok() {
            #[cfg(debug_assertions)]
            self.assert_valid();
        }
        ret
    }

    /// Appends new [`UnresolvedPackageGroup`] to this [`Resolve`], creating a
    /// fully resolved package with no dangling references.
    ///
    /// Any dependency resolution error or otherwise world-elaboration error
    /// will be returned here, if successful a package identifier is returned
    /// which corresponds to the package that was just inserted.
    ///
    /// On error, `self` may be partially modified and should not be reused.
    pub fn push_group(
        &mut self,
        unresolved_group: UnresolvedPackageGroup,
    ) -> ResolveResult<PackageId> {
        let (pkg_id, _) = self.sort_unresolved_packages(unresolved_group, Vec::new())?;
        Ok(pkg_id)
    }

    /// Appends a main [`UnresolvedPackageGroup`] and its dependencies to this
    /// [`Resolve`] in a single call, topologically sorting them internally.
    ///
    /// This is useful when you have a package and its local dependencies
    /// available as in-memory [`UnresolvedPackageGroup`]s and want dependency
    /// ordering and cycle detection handled automatically.
    ///
    /// The returned [`PackageId`] corresponds to `main`.
    ///
    /// On error, `self` may be partially modified and should not be reused.
    pub fn push_groups(
        &mut self,
        main: UnresolvedPackageGroup,
        deps: Vec<UnresolvedPackageGroup>,
    ) -> ResolveResult<PackageId> {
        let (pkg_id, _) = self.sort_unresolved_packages(main, deps)?;
        Ok(pkg_id)
    }

    /// Convenience method for combining [`SourceMap`] and [`Resolve::push_group`].
    ///
    /// The `path` provided is used for error messages but otherwise is not
    /// read. This method does not touch the filesystem. The `contents` provided
    /// are the contents of a WIT package.
    pub fn push_source(&mut self, path: &str, contents: &str) -> anyhow::Result<PackageId> {
        let mut map = SourceMap::default();
        map.push_str(path, contents);
        let group = self.parse_source_map(map)?;
        Ok(self.push_group(group)?)
    }

    /// Parses `map` into an [`UnresolvedPackageGroup`].
    ///
    /// On parse failure the local [`SourceMap`] is merged into
    /// [`Resolve::source_map`] (with the typed [`crate::ParseError`]'s spans
    /// adjusted) so the downcasted error remains renderable against
    /// `self.source_map`.
    pub(crate) fn parse_source_map(
        &mut self,
        map: SourceMap,
    ) -> anyhow::Result<UnresolvedPackageGroup> {
        map.parse().map_err(|(map, mut e)| {
            let offset = self.source_map.append(map);
            e.adjust_spans(offset);
            e.into()
        })
    }

    /// Renders a span as a human-readable location string (e.g., "file.wit:10:5").
    pub fn render_location(&self, span: Span) -> String {
        self.source_map.render_location(span)
    }

    /// Renders an error returned by this [`Resolve`]'s `push_*` methods with
    /// source context (file:line:col + snippet).
    #[cfg(feature = "std")]
    pub fn render_error(&self, err: &anyhow::Error) -> String {
        crate::render_anyhow_error(err, &self.source_map)
    }

    pub fn all_bits_valid(&self, ty: &Type) -> bool {
        match ty {
            Type::U8
            | Type::S8
            | Type::U16
            | Type::S16
            | Type::U32
            | Type::S32
            | Type::U64
            | Type::S64
            | Type::F32
            | Type::F64 => true,

            Type::Bool | Type::Char | Type::String | Type::ErrorContext => false,

            Type::Id(id) => match &self.types[*id].kind {
                TypeDefKind::List(_)
                | TypeDefKind::Map(_, _)
                | TypeDefKind::Variant(_)
                | TypeDefKind::Enum(_)
                | TypeDefKind::Option(_)
                | TypeDefKind::Result(_)
                | TypeDefKind::Future(_)
                | TypeDefKind::Stream(_) => false,
                TypeDefKind::Type(t) | TypeDefKind::FixedLengthList(t, ..) => {
                    self.all_bits_valid(t)
                }

                TypeDefKind::Handle(h) => match h {
                    crate::Handle::Own(_) => true,
                    crate::Handle::Borrow(_) => true,
                },

                TypeDefKind::Resource => false,
                TypeDefKind::Record(r) => r.fields.iter().all(|f| self.all_bits_valid(&f.ty)),
                TypeDefKind::Tuple(t) => t.types.iter().all(|t| self.all_bits_valid(t)),

                // FIXME: this could perhaps be `true` for multiples-of-32 but
                // seems better to probably leave this as unconditionally
                // `false` for now, may want to reconsider later?
                TypeDefKind::Flags(_) => false,

                TypeDefKind::Unknown => unreachable!(),
            },
        }
    }

    /// Merges all the contents of a different `Resolve` into this one. The
    /// `Remap` structure returned provides a mapping from all old indices to
    /// new indices
    ///
    /// This operation can fail if `resolve` disagrees with `self` about the
    /// packages being inserted. Otherwise though this will additionally attempt
    /// to "union" packages found in `resolve` with those found in `self`.
    /// Unioning packages is keyed on the name/url of packages for those with
    /// URLs present. If found then it's assumed that both `Resolve` instances
    /// were originally created from the same contents and are two views
    /// of the same package.
    pub fn merge(&mut self, resolve: Resolve) -> anyhow::Result<Remap> {
        log::trace!(
            "merging {} packages into {} packages",
            resolve.packages.len(),
            self.packages.len()
        );

        let mut map = MergeMap::new(&resolve, &self);
        map.build()?;
        let MergeMap {
            package_map,
            interface_map,
            type_map,
            world_map,
            interfaces_to_add,
            worlds_to_add,
            ..
        } = map;

        // With a set of maps from ids in `resolve` to ids in `self` the next
        // operation is to start moving over items and building a `Remap` to
        // update ids.
        //
        // Each component field of `resolve` is moved into `self` so long as
        // its ID is not within one of the maps above. If it's present in a map
        // above then that means the item is already present in `self` so a new
        // one need not be added. If it's not present in a map that means it's
        // not present in `self` so it must be added to an arena.
        //
        // When adding an item to an arena one of the `remap.update_*` methods
        // is additionally called to update all identifiers from pointers within
        // `resolve` to becoming pointers within `self`.
        //
        // Altogether this should weave all the missing items in `self` from
        // `resolve` into one structure while updating all identifiers to
        // be local within `self`.

        let mut remap = Remap::default();
        let Resolve {
            types,
            worlds,
            interfaces,
            packages,
            package_names,
            features: _,
            source_map,
            ..
        } = resolve;

        let span_offset = self.source_map.append(source_map);

        let mut moved_types = Vec::new();
        for (id, mut ty) in types {
            let new_id = match type_map.get(&id).copied() {
                Some(id) => {
                    update_stability(&ty.stability, &mut self.types[id].stability, ty.span)?;
                    id
                }
                None => {
                    log::debug!("moving type {:?}", ty.name);
                    moved_types.push(id);
                    remap.update_typedef(self, &mut ty, Default::default())?;
                    ty.adjust_spans(span_offset);
                    self.types.alloc(ty)
                }
            };
            assert_eq!(remap.types.len(), id.index());
            remap.types.push(Some(new_id));
        }

        let mut moved_interfaces = Vec::new();
        for (id, mut iface) in interfaces {
            let new_id = match interface_map.get(&id).copied() {
                Some(into_id) => {
                    update_stability(
                        &iface.stability,
                        &mut self.interfaces[into_id].stability,
                        iface.span,
                    )?;

                    // Add any extra types from `from`'s interface that
                    // don't exist in `into`'s interface. These types were
                    // already moved as new types above (since they weren't
                    // in `type_map`), but they still need to be registered
                    // in the target interface's `types` map.
                    for (name, from_type_id) in iface.types.iter() {
                        if self.interfaces[into_id].types.contains_key(name) {
                            continue;
                        }
                        let new_type_id = remap.map_type(*from_type_id, Default::default())?;
                        self.interfaces[into_id]
                            .types
                            .insert(name.clone(), new_type_id);
                    }

                    // Add any extra functions from `from`'s interface that
                    // don't exist in `into`'s interface. These need their
                    // type references remapped and spans adjusted.
                    let extra_funcs: Vec<_> = iface
                        .functions
                        .into_iter()
                        .filter(|(name, _)| {
                            !self.interfaces[into_id]
                                .functions
                                .contains_key(name.as_str())
                        })
                        .collect();
                    for (name, mut func) in extra_funcs {
                        remap.update_function(self, &mut func, Default::default())?;
                        func.adjust_spans(span_offset);
                        self.interfaces[into_id].functions.insert(name, func);
                    }

                    into_id
                }
                None => {
                    log::debug!("moving interface {:?}", iface.name);
                    moved_interfaces.push(id);
                    remap.update_interface(self, &mut iface)?;
                    iface.adjust_spans(span_offset);
                    self.interfaces.alloc(iface)
                }
            };
            assert_eq!(remap.interfaces.len(), id.index());
            remap.interfaces.push(Some(new_id));
        }

        let mut moved_worlds = Vec::new();
        for (id, mut world) in worlds {
            let new_id = match world_map.get(&id).copied() {
                Some(world_id) => {
                    update_stability(
                        &world.stability,
                        &mut self.worlds[world_id].stability,
                        world.span,
                    )?;
                    for from_import in world.imports.iter() {
                        Resolve::update_world_imports_stability(
                            from_import,
                            &mut self.worlds[world_id].imports,
                            &interface_map,
                        )?;
                    }
                    for from_export in world.exports.iter() {
                        Resolve::update_world_imports_stability(
                            from_export,
                            &mut self.worlds[world_id].exports,
                            &interface_map,
                        )?;
                    }
                    world_id
                }
                None => {
                    log::debug!("moving world {}", world.name);
                    moved_worlds.push(id);
                    let mut update =
                        |map: &mut IndexMap<WorldKey, WorldItem>| -> anyhow::Result<_> {
                            for (mut name, mut item) in mem::take(map) {
                                remap.update_world_key(&mut name, Default::default())?;
                                match &mut item {
                                    WorldItem::Function(f) => {
                                        remap.update_function(self, f, Default::default())?
                                    }
                                    WorldItem::Interface { id, .. } => {
                                        *id = remap.map_interface(*id, Default::default())?;
                                    }
                                    WorldItem::Type { id, .. } => {
                                        *id = remap.map_type(*id, Default::default())?
                                    }
                                }
                                map.insert(name, item);
                            }
                            Ok(())
                        };
                    update(&mut world.imports)?;
                    update(&mut world.exports)?;
                    world.adjust_spans(span_offset);
                    self.worlds.alloc(world)
                }
            };
            assert_eq!(remap.worlds.len(), id.index());
            remap.worlds.push(Some(new_id));
        }

        for (id, mut pkg) in packages {
            let new_id = match package_map.get(&id).copied() {
                Some(id) => id,
                None => {
                    for (_, id) in pkg.interfaces.iter_mut() {
                        *id = remap.map_interface(*id, Default::default())?;
                    }
                    for (_, id) in pkg.worlds.iter_mut() {
                        *id = remap.map_world(*id, Default::default())?;
                    }
                    self.packages.alloc(pkg)
                }
            };
            assert_eq!(remap.packages.len(), id.index());
            remap.packages.push(new_id);
        }

        for (name, id) in package_names {
            let id = remap.packages[id.index()];
            if let Some(prev) = self.package_names.insert(name, id) {
                assert_eq!(prev, id);
            }
        }

        // Fixup all "parent" links now.
        //
        // Note that this is only done for items that are actually moved from
        // `resolve` into `self`, which is tracked by the various `moved_*`
        // lists built incrementally above. The ids in the `moved_*` lists
        // are ids within `resolve`, so they're translated through `remap` to
        // ids within `self`.
        for id in moved_worlds {
            let id = remap.map_world(id, Default::default())?;
            if let Some(pkg) = self.worlds[id].package.as_mut() {
                *pkg = remap.packages[pkg.index()];
            }
        }
        for id in moved_interfaces {
            let id = remap.map_interface(id, Default::default())?;
            if let Some(pkg) = self.interfaces[id].package.as_mut() {
                *pkg = remap.packages[pkg.index()];
            }
            if let Some(clone_of) = self.interfaces[id].clone_of.as_mut() {
                *clone_of = remap.map_interface(*clone_of, Default::default())?;
            }
        }
        for id in moved_types {
            let id = remap.map_type(id, Default::default())?;
            match &mut self.types[id].owner {
                TypeOwner::Interface(id) => *id = remap.map_interface(*id, Default::default())?,
                TypeOwner::World(id) => *id = remap.map_world(*id, Default::default())?,
                TypeOwner::None => {}
            }
        }

        // And finally process items that were present in `resolve` but were
        // not present in `self`. This is only done for merged packages as
        // documents may be added to `self.documents` but wouldn't otherwise be
        // present in the `documents` field of the corresponding package.
        for (name, pkg, iface) in interfaces_to_add {
            let prev = self.packages[pkg]
                .interfaces
                .insert(name, remap.map_interface(iface, Default::default())?);
            assert!(prev.is_none());
        }
        for (name, pkg, world) in worlds_to_add {
            let prev = self.packages[pkg]
                .worlds
                .insert(name, remap.map_world(world, Default::default())?);
            assert!(prev.is_none());
        }

        log::trace!("now have {} packages", self.packages.len());

        // Re-elaborate all worlds after the merge. Merging may have added
        // extra types to existing interfaces that introduce new interface
        // dependencies not yet present in the world's imports.
        let world_ids: Vec<_> = self.worlds.iter().map(|(id, _)| id).collect();
        for world_id in world_ids {
            let world_span = self.worlds[world_id].span;
            self.elaborate_world(world_id, world_span)?;
        }

        #[cfg(debug_assertions)]
        self.assert_valid();
        Ok(remap)
    }

    fn update_world_imports_stability(
        from_item: (&WorldKey, &WorldItem),
        into_items: &mut IndexMap<WorldKey, WorldItem>,
        interface_map: &HashMap<Id<Interface>, Id<Interface>>,
    ) -> anyhow::Result<()> {
        match from_item.0 {
            WorldKey::Name(_) => {
                // No stability info to update here, only updating import/include stability
                Ok(())
            }
            key @ WorldKey::Interface(_) => {
                let new_key = MergeMap::map_name(key, interface_map);
                if let Some(into) = into_items.get_mut(&new_key) {
                    match (from_item.1, into) {
                        (
                            WorldItem::Interface {
                                id: aid,
                                stability: astability,
                                span: aspan,
                                ..
                            },
                            WorldItem::Interface {
                                id: bid,
                                stability: bstability,
                                ..
                            },
                        ) => {
                            let aid = interface_map.get(aid).copied().unwrap_or(*aid);
                            assert_eq!(aid, *bid);
                            update_stability(astability, bstability, *aspan)?;
                            Ok(())
                        }
                        _ => unreachable!(),
                    }
                } else {
                    // we've already matched all the imports/exports by the time we are calling this
                    // so this is unreachable since we should always find the item
                    unreachable!()
                }
            }
        }
    }

    /// Merges the world `from` into the world `into`.
    ///
    /// This will attempt to merge one world into another, unioning all of its
    /// imports and exports together. This is an operation performed by
    /// `wit-component`, for example where two different worlds from two
    /// different libraries were linked into the same core wasm file and are
    /// producing a singular world that will be the final component's
    /// interface.
    ///
    /// During the merge operation, some of the types and/or interfaces in
    /// `from` might need to be cloned so that backreferences point to `into`
    /// instead of `from`.  Any such clones will be added to `clone_maps`.
    ///
    /// This operation can fail if the imports/exports overlap.
    pub fn merge_worlds(
        &mut self,
        from: WorldId,
        into: WorldId,
        clone_maps: &mut CloneMaps,
    ) -> anyhow::Result<()> {
        let mut new_imports = Vec::new();
        let mut new_exports = Vec::new();

        let from_world = &self.worlds[from];
        let into_world = &self.worlds[into];

        log::trace!("merging {} into {}", from_world.name, into_world.name);

        // First walk over all the imports of `from` world and figure out what
        // to do with them.
        //
        // If the same item exists in `from` and `into` then merge it together
        // below with `merge_world_item` which basically asserts they're the
        // same. Otherwise queue up a new import since if `from` has more
        // imports than `into` then it's fine to add new imports.
        for (name, from_import) in from_world.imports.iter() {
            let name_str = self.name_world_key(name);
            match into_world.imports.get(name) {
                Some(into_import) => {
                    log::trace!("info/from shared import on `{name_str}`");
                    self.merge_world_item(from_import, into_import)
                        .with_context(|| format!("failed to merge world import {name_str}"))?;
                }
                None => {
                    log::trace!("new import: `{name_str}`");
                    new_imports.push((name.clone(), from_import.clone()));
                }
            }
        }

        // Build a set of interfaces which are required to be imported because
        // of `into`'s exports. This set is then used below during
        // `ensure_can_add_world_export`.
        //
        // This is the set of interfaces which exports depend on that are
        // themselves not exports.
        let mut must_be_imported = HashMap::new();
        for (key, export) in into_world.exports.iter() {
            for dep in self.world_item_direct_deps(export) {
                if into_world.exports.contains_key(&WorldKey::Interface(dep)) {
                    continue;
                }
                self.foreach_interface_dep(dep, &mut |id| {
                    must_be_imported.insert(id, key.clone());
                });
            }
        }

        // Next walk over exports of `from` and process these similarly to
        // imports.
        for (name, from_export) in from_world.exports.iter() {
            let name_str = self.name_world_key(name);
            match into_world.exports.get(name) {
                Some(into_export) => {
                    log::trace!("info/from shared export on `{name_str}`");
                    self.merge_world_item(from_export, into_export)
                        .with_context(|| format!("failed to merge world export {name_str}"))?;
                }
                None => {
                    log::trace!("new export `{name_str}`");
                    // See comments in `ensure_can_add_world_export` for why
                    // this is slightly different than imports.
                    self.ensure_can_add_world_export(
                        into_world,
                        name,
                        from_export,
                        &must_be_imported,
                    )
                    .with_context(|| {
                        format!("failed to add export `{}`", self.name_world_key(name))
                    })?;
                    new_exports.push((name.clone(), from_export.clone()));
                }
            }
        }

        // For all the new imports and exports they may need to be "cloned" to
        // be able to belong to the new world. For example:
        //
        // * Anonymous interfaces have a `package` field which points to the
        //   package of the containing world, but `from` and `into` may not be
        //   in the same package.
        //
        // * Type imports have an `owner` field that point to `from`, but they
        //   now need to point to `into` instead.
        //
        // Cloning is no trivial task, however, so cloning is delegated to a
        // submodule to perform a "deep" clone and copy items into new arena
        // entries as necessary.
        let mut cloner = clone::Cloner::new(
            self,
            clone_maps,
            TypeOwner::World(from),
            TypeOwner::World(into),
        );
        cloner.register_world_type_overlap(from, into);
        for (name, item) in new_imports.iter_mut().chain(&mut new_exports) {
            cloner.world_item(name, item);
        }

        // Insert any new imports and new exports found first.
        let into_world = &mut self.worlds[into];
        for (name, import) in new_imports {
            let prev = into_world.imports.insert(name, import);
            assert!(prev.is_none());
        }
        for (name, export) in new_exports {
            let prev = into_world.exports.insert(name, export);
            assert!(prev.is_none());
        }

        #[cfg(debug_assertions)]
        self.assert_valid();
        Ok(())
    }

    fn merge_world_item(&self, from: &WorldItem, into: &WorldItem) -> anyhow::Result<()> {
        let mut map = MergeMap::new(self, self);
        match (from, into) {
            (WorldItem::Interface { id: from, .. }, WorldItem::Interface { id: into, .. }) => {
                // If these imports are the same that can happen, for
                // example, when both worlds to `import foo:bar/baz;`. That
                // foreign interface will point to the same interface within
                // `Resolve`.
                if from == into {
                    return Ok(());
                }

                // .. otherwise this MUST be a case of
                // `import foo: interface { ... }`. If `from != into` but
                // both `from` and `into` have the same name then the
                // `WorldKey::Interface` case is ruled out as otherwise
                // they'd have different names.
                //
                // In the case of an anonymous interface all we can do is
                // ensure that the interfaces both match, so use `MergeMap`
                // for that.
                map.build_interface(*from, *into)
                    .context("failed to merge interfaces")?;
            }

            // Like `WorldKey::Name` interfaces for functions and types the
            // structure is asserted to be the same.
            (WorldItem::Function(from), WorldItem::Function(into)) => {
                map.build_function(from, into)
                    .context("failed to merge functions")?;
            }
            (WorldItem::Type { id: from, .. }, WorldItem::Type { id: into, .. }) => {
                map.build_type_id(*from, *into)
                    .context("failed to merge types")?;
            }

            // Kind-level mismatches are caught here.
            (WorldItem::Interface { .. }, _)
            | (WorldItem::Function { .. }, _)
            | (WorldItem::Type { .. }, _) => {
                bail!("different kinds of items");
            }
        }
        assert!(map.interfaces_to_add.is_empty());
        assert!(map.worlds_to_add.is_empty());
        Ok(())
    }

    /// This method ensures that the world export of `name` and `item` can be
    /// added to the world `into` without changing the meaning of `into`.
    ///
    /// All dependencies of world exports must either be:
    ///
    /// * An export themselves
    /// * An import with all transitive dependencies of the import also imported
    ///
    /// It's not possible to depend on an import which then also depends on an
    /// export at some point, for example. This method ensures that if `name`
    /// and `item` are added that this property is upheld.
    fn ensure_can_add_world_export(
        &self,
        into: &World,
        name: &WorldKey,
        item: &WorldItem,
        must_be_imported: &HashMap<InterfaceId, WorldKey>,
    ) -> anyhow::Result<()> {
        assert!(!into.exports.contains_key(name));
        let name = self.name_world_key(name);

        // First make sure that all of this item's dependencies are either
        // exported or the entire chain of imports rooted at that dependency are
        // all imported.
        for dep in self.world_item_direct_deps(item) {
            if into.exports.contains_key(&WorldKey::Interface(dep)) {
                continue;
            }
            self.ensure_not_exported(into, dep)
                .with_context(|| format!("failed validating export of `{name}`"))?;
        }

        // Second make sure that this item, if it's an interface, will not alter
        // the meaning of the preexisting world by ensuring that it's not in the
        // set of "must be imported" items.
        if let WorldItem::Interface { id, .. } = item {
            if let Some(export) = must_be_imported.get(id) {
                let export_name = self.name_world_key(export);
                bail!(
                    "export `{export_name}` depends on `{name}` \
                     previously as an import which will change meaning \
                     if `{name}` is added as an export"
                );
            }
        }

        Ok(())
    }

    fn ensure_not_exported(&self, world: &World, id: InterfaceId) -> anyhow::Result<()> {
        let key = WorldKey::Interface(id);
        let name = self.name_world_key(&key);
        if world.exports.contains_key(&key) {
            bail!(
                "world exports `{name}` but it's also transitively used by an \
                     import \
                   which means that this is not valid"
            )
        }
        for dep in self.interface_direct_deps(id) {
            self.ensure_not_exported(world, dep)
                .with_context(|| format!("failed validating transitive import dep `{name}`"))?;
        }
        Ok(())
    }

    /// Returns an iterator of all the direct interface dependencies of this
    /// `item`.
    ///
    /// Note that this doesn't include transitive dependencies, that must be
    /// followed manually.
    fn world_item_direct_deps(&self, item: &WorldItem) -> impl Iterator<Item = InterfaceId> + '_ {
        let mut interface = None;
        let mut ty = None;
        match item {
            WorldItem::Function(_) => {}
            WorldItem::Type { id, .. } => ty = Some(*id),
            WorldItem::Interface { id, .. } => interface = Some(*id),
        }

        interface
            .into_iter()
            .flat_map(move |id| self.interface_direct_deps(id))
            .chain(ty.and_then(|t| self.type_interface_dep(t)))
    }

    /// Invokes `f` with `id` and all transitive interface dependencies of `id`.
    ///
    /// Note that `f` may be called with the same id multiple times.
    fn foreach_interface_dep(&self, id: InterfaceId, f: &mut dyn FnMut(InterfaceId)) {
        self._foreach_interface_dep(id, f, &mut HashSet::new())
    }

    // Internal detail of `foreach_interface_dep` which uses a hash map to prune
    // the visit tree to ensure that this doesn't visit an exponential number of
    // interfaces.
    fn _foreach_interface_dep(
        &self,
        id: InterfaceId,
        f: &mut dyn FnMut(InterfaceId),
        visited: &mut HashSet<InterfaceId>,
    ) {
        if !visited.insert(id) {
            return;
        }
        f(id);
        for dep in self.interface_direct_deps(id) {
            self._foreach_interface_dep(dep, f, visited);
        }
    }

    /// Returns the ID of the specified `interface`.
    ///
    /// Returns `None` for unnamed interfaces.
    pub fn id_of(&self, interface: InterfaceId) -> Option<String> {
        let interface = &self.interfaces[interface];
        Some(self.id_of_name(interface.package.unwrap(), interface.name.as_ref()?))
    }

    /// Returns the "canonicalized interface name" of `interface`.
    ///
    /// Returns `None` for unnamed interfaces. See `BuildTargets.md` in the
    /// upstream component model repository for more information about this.
    pub fn canonicalized_id_of(&self, interface: InterfaceId) -> Option<String> {
        let interface = &self.interfaces[interface];
        Some(self.canonicalized_id_of_name(interface.package.unwrap(), interface.name.as_ref()?))
    }

    /// Helper to rename a world and update the package's world map.
    ///
    /// Used by both [`Resolve::importize`] and [`Resolve::exportize`] to
    /// rename the world to avoid confusion with the original world name.
    fn rename_world(
        &mut self,
        world_id: WorldId,
        out_world_name: Option<String>,
        default_suffix: &str,
    ) {
        let world = &mut self.worlds[world_id];
        let pkg = &mut self.packages[world.package.unwrap()];
        pkg.worlds.shift_remove(&world.name);
        if let Some(name) = out_world_name {
            world.name = name.clone();
            pkg.worlds.insert(name, world_id);
        } else {
            world.name.push_str(default_suffix);
            pkg.worlds.insert(world.name.clone(), world_id);
        }
    }

    /// Convert a world to an "importized" version where the world is updated
    /// in-place to reflect what it would look like to be imported.
    ///
    /// This is a transformation which is used as part of the process of
    /// importing a component today. For example when a component depends on
    /// another component this is useful for generating WIT which can be use to
    /// represent the component being imported. The general idea is that this
    /// function will update the `world_id` specified such it imports the
    /// functionality that it previously exported. The world will be left with
    /// no exports.
    ///
    /// This world is then suitable for merging into other worlds or generating
    /// bindings in a context that is importing the original world. This
    /// is intended to be used as part of language tooling when depending on
    /// other components.
    pub fn importize(
        &mut self,
        world_id: WorldId,
        out_world_name: Option<String>,
    ) -> anyhow::Result<()> {
        self.rename_world(world_id, out_world_name, "-importized");

        // Trim all non-type definitions from imports. Types can be used by
        // exported functions, for example, so they're preserved.
        let world = &mut self.worlds[world_id];
        world.imports.retain(|_, item| match item {
            WorldItem::Type { .. } => true,
            _ => false,
        });

        for (name, export) in mem::take(&mut world.exports) {
            match (name.clone(), world.imports.insert(name, export)) {
                // no previous item? this insertion was ok
                (_, None) => {}

                // cannot overwrite an import with an export
                (WorldKey::Name(name), Some(_)) => {
                    bail!("world export `{name}` conflicts with import of same name");
                }

                // Exports already don't overlap each other and the only imports
                // preserved above were types so this shouldn't be reachable.
                (WorldKey::Interface(_), _) => unreachable!(),
            }
        }

        // Fill out any missing transitive interface imports by elaborating this
        // world which does that for us.
        let world_span = world.span;
        self.elaborate_world(world_id, world_span)?;

        #[cfg(debug_assertions)]
        self.assert_valid();
        Ok(())
    }

    /// Convert a world to an "exportized" version where the world is updated
    /// in-place to reflect what it would look like to be exported.
    ///
    /// This is the inverse of [`Resolve::importize`]. The general idea is that
    /// this function will update the `world_id` specified such that it exports
    /// the functionality that it previously imported. The world will be left
    /// with no imports (except for transitive interface dependencies which may
    /// be needed by exported interfaces).
    ///
    /// An optional `filter` can be provided to control which imports are moved.
    /// When `Some`, only imports for which the filter returns `true` are moved
    /// to exports; remaining imports are left as-is. When `None`, all imports
    /// are moved.
    ///
    /// This world is then suitable for merging into other worlds or generating
    /// bindings in a context that is exporting the original world. This is
    /// intended to be used as part of language tooling when implementing
    /// components.
    pub fn exportize(
        &mut self,
        world_id: WorldId,
        out_world_name: Option<String>,
        filter: Option<&dyn Fn(&WorldKey, &WorldItem) -> bool>,
    ) -> anyhow::Result<()> {
        self.rename_world(world_id, out_world_name, "-exportized");

        let world = &mut self.worlds[world_id];
        world.exports.clear();

        let old_imports = mem::take(&mut world.imports);
        for (name, import) in old_imports {
            let should_move = match &filter {
                Some(f) => f(&name, &import),
                None => true,
            };
            if should_move {
                world.exports.insert(name, import);
            } else {
                world.imports.insert(name, import);
            }
        }

        // Fill out any missing transitive interface imports by elaborating this
        // world which does that for us.
        let world_span = world.span;
        self.elaborate_world(world_id, world_span)?;

        #[cfg(debug_assertions)]
        self.assert_valid();
        Ok(())
    }

    /// Returns the ID of the specified `name` within the `pkg`.
    pub fn id_of_name(&self, pkg: PackageId, name: &str) -> String {
        let package = &self.packages[pkg];
        let mut base = String::new();
        base.push_str(&package.name.namespace);
        base.push_str(":");
        base.push_str(&package.name.name);
        base.push_str("/");
        base.push_str(name);
        if let Some(version) = &package.name.version {
            base.push_str(&format!("@{version}"));
        }
        base
    }

    /// Returns the "canonicalized interface name" of the specified `name`
    /// within the `pkg`.
    ///
    /// See `BuildTargets.md` in the upstream component model repository for
    /// more information about this.
    pub fn canonicalized_id_of_name(&self, pkg: PackageId, name: &str) -> String {
        let package = &self.packages[pkg];
        let mut base = String::new();
        base.push_str(&package.name.namespace);
        base.push_str(":");
        base.push_str(&package.name.name);
        base.push_str("/");
        base.push_str(name);
        if let Some(version) = &package.name.version {
            base.push_str("@");
            let string = PackageName::version_compat_track_string(version);
            base.push_str(&string);
        }
        base
    }

    /// Selects a world from among the packages in a `Resolve`.
    ///
    /// A `Resolve` may have many packages, each with many worlds. Many WIT
    /// tools need a specific world to operate on. This function chooses a
    /// world, failing if the choice is ambiguous.
    ///
    /// `main_packages` provides the package IDs returned by
    /// [`push_path`](Resolve::push_path), [`push_dir`](Resolve::push_dir),
    /// [`push_file`](Resolve::push_file), [`push_group`](Resolve::push_group),
    /// and [`push_str`](Resolve::push_str), which are the "main packages",
    /// as distinguished from any packages nested inside them.
    ///
    /// `world` is a world name such as from a `--world` command-line option or
    /// a `world:` macro parameter. `world` can be:
    ///
    /// * A kebab-name of a world, for example `"the-world"`. It is resolved
    ///   within the "main package", if there is exactly one.
    ///
    /// * An ID-based form of a world, for example `"wasi:http/proxy"`. Note
    ///   that a version does not need to be specified in this string if
    ///   there's only one package of the same name and it has a version. In
    ///   this situation the version can be omitted.
    ///
    /// * `None`. If there's exactly one "main package" and it contains exactly
    ///   one world, that world is chosen.
    ///
    /// If successful, the chosen `WorldId` is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use anyhow::Result;
    /// use wit_parser::Resolve;
    ///
    /// fn main() -> Result<()> {
    ///     let mut resolve = Resolve::default();
    ///
    ///     // If there's a single package and only one world, that world is
    ///     // the obvious choice.
    ///     let wit1 = resolve.push_str(
    ///         "./my-test.wit",
    ///         r#"
    ///             package example:wit1;
    ///
    ///             world foo {
    ///                 // ...
    ///             }
    ///         "#,
    ///     )?;
    ///     assert!(resolve.select_world(&[wit1], None).is_ok());
    ///
    ///     // If there are multiple packages, we need to be told which package
    ///     // to use, either by a "main package" or by a fully-qualified name.
    ///     let wit2 = resolve.push_str(
    ///         "./my-test.wit",
    ///         r#"
    ///             package example:wit2;
    ///
    ///             world foo { /* ... */ }
    ///         "#,
    ///     )?;
    ///     assert!(resolve.select_world(&[wit1, wit2], None).is_err());
    ///     assert!(resolve.select_world(&[wit1, wit2], Some("foo")).is_err());
    ///     // Fix: use fully-qualified names.
    ///     assert!(resolve.select_world(&[wit1, wit2], Some("example:wit1/foo")).is_ok());
    ///     assert!(resolve.select_world(&[wit1, wit2], Some("example:wit2/foo")).is_ok());
    ///
    ///     // If a package has multiple worlds, then we can't guess the world
    ///     // even if we know the package.
    ///     let wit3 = resolve.push_str(
    ///         "./my-test.wit",
    ///         r#"
    ///             package example:wit3;
    ///
    ///             world foo { /* ... */ }
    ///
    ///             world bar { /* ... */ }
    ///         "#,
    ///     )?;
    ///     assert!(resolve.select_world(&[wit3], None).is_err());
    ///     // Fix: pick between "foo" and "bar" here.
    ///     assert!(resolve.select_world(&[wit3], Some("foo")).is_ok());
    ///
    ///     // When selecting with a version it's ok to drop the version when
    ///     // there's only a single copy of that package in `Resolve`.
    ///     let wit5_1 = resolve.push_str(
    ///         "./my-test.wit",
    ///         r#"
    ///             package example:wit5@1.0.0;
    ///
    ///             world foo { /* ... */ }
    ///         "#,
    ///     )?;
    ///     assert!(resolve.select_world(&[wit5_1], Some("foo")).is_ok());
    ///     assert!(resolve.select_world(&[wit5_1], Some("example:wit5/foo")).is_ok());
    ///
    ///     // However when a single package has multiple versions in a resolve
    ///     // it's required to specify the version to select which one.
    ///     let wit5_2 = resolve.push_str(
    ///         "./my-test.wit",
    ///         r#"
    ///             package example:wit5@2.0.0;
    ///
    ///             world foo { /* ... */ }
    ///         "#,
    ///     )?;
    ///     assert!(resolve.select_world(&[wit5_1, wit5_2], Some("example:wit5/foo")).is_err());
    ///     // Fix: Pass explicit versions.
    ///     assert!(resolve.select_world(&[wit5_1, wit5_2], Some("example:wit5/foo@1.0.0")).is_ok());
    ///     assert!(resolve.select_world(&[wit5_1, wit5_2], Some("example:wit5/foo@2.0.0")).is_ok());
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn select_world(
        &self,
        main_packages: &[PackageId],
        world: Option<&str>,
    ) -> anyhow::Result<WorldId> {
        // Determine if `world` is a kebab-name or an ID.
        let world_path = match world {
            Some(world) => Some(
                parse_use_path(world)
                    .with_context(|| format!("failed to parse world specifier `{world}`"))?,
            ),
            None => None,
        };

        match world_path {
            // We have a world path. If needed, pick a package to resolve it in.
            Some(world_path) => {
                let (pkg, world_name) = match (main_packages, world_path) {
                    // We have no main packages; fail.
                    ([], _) => bail!("No main packages defined"),

                    // We have exactly one main package.
                    ([main_package], ParsedUsePath::Name(name)) => (*main_package, name),

                    // We have more than one main package; fail.
                    (_, ParsedUsePath::Name(_name)) => {
                        bail!(
                            "There are multiple main packages; a world must be explicitly chosen:{}",
                            self.worlds
                                .iter()
                                .map(|world| format!(
                                    "\n  {}",
                                    self.id_of_name(world.1.package.unwrap(), &world.1.name)
                                ))
                                .collect::<String>()
                        )
                    }

                    // The world name is fully-qualified.
                    (_, ParsedUsePath::Package(pkg, world_name)) => {
                        let pkg = match self.package_names.get(&pkg) {
                            Some(pkg) => *pkg,
                            None => {
                                let mut candidates =
                                    self.package_names.iter().filter(|(name, _)| {
                                        pkg.version.is_none()
                                            && pkg.name == name.name
                                            && pkg.namespace == name.namespace
                                            && name.version.is_some()
                                    });
                                let candidate = candidates.next();
                                if let Some((c2, _)) = candidates.next() {
                                    let (c1, _) = candidate.unwrap();
                                    bail!(
                                        "package name `{pkg}` is available at both \
                                    versions {} and {} but which is not specified",
                                        c1.version.as_ref().unwrap(),
                                        c2.version.as_ref().unwrap(),
                                    );
                                }
                                match candidate {
                                    Some((_, id)) => *id,
                                    None => bail!("unknown package `{pkg}`"),
                                }
                            }
                        };
                        (pkg, world_name.to_string())
                    }
                };

                // Now that we've picked the package, resolve the world name.
                let pkg = &self.packages[pkg];
                pkg.worlds.get(&world_name).copied().ok_or_else(|| {
                    anyhow!("World `{world_name}` not found in package `{}`", pkg.name)
                })
            }

            // With no specified `world`, try to find a single obvious world.
            None => match main_packages {
                [] => bail!("No main packages defined"),

                // Check for exactly one main package with exactly one world.
                [main_package] => {
                    let pkg = &self.packages[*main_package];
                    match pkg.worlds.len() {
                        0 => bail!("The main package `{}` contains no worlds", pkg.name),
                        1 => Ok(pkg.worlds[0]),
                        _ => bail!(
                            "There are multiple worlds in `{}`; one must be explicitly chosen:{}",
                            pkg.name,
                            pkg.worlds
                                .values()
                                .map(|world| format!(
                                    "\n  {}",
                                    self.id_of_name(*main_package, &self.worlds[*world].name)
                                ))
                                .collect::<String>()
                        ),
                    }
                }

                // Multiple main packages and no world name; fail.
                _ => {
                    bail!(
                        "There are multiple main packages; a world must be explicitly chosen:{}",
                        self.worlds
                            .iter()
                            .map(|world| format!(
                                "\n  {}",
                                self.id_of_name(world.1.package.unwrap(), &world.1.name)
                            ))
                            .collect::<String>()
                    )
                }
            },
        }
    }

    /// Assigns a human readable name to the `WorldKey` specified.
    pub fn name_world_key(&self, key: &WorldKey) -> String {
        match key {
            WorldKey::Name(s) => s.to_string(),
            WorldKey::Interface(i) => self.id_of(*i).expect("unexpected anonymous interface"),
        }
    }

    /// Same as [`Resolve::name_world_key`] except that `WorldKey::Interfaces`
    /// uses [`Resolve::canonicalized_id_of`].
    pub fn name_canonicalized_world_key(&self, key: &WorldKey) -> String {
        match key {
            WorldKey::Name(s) => s.to_string(),
            WorldKey::Interface(i) => self
                .canonicalized_id_of(*i)
                .expect("unexpected anonymous interface"),
        }
    }

    /// Returns the component model `implements` value for the world import of
    /// `key` and `item`.
    ///
    /// See the component model explainer and 🏷️ for more information on this feature.
    pub fn implements_value(&self, key: &WorldKey, item: &WorldItem) -> Option<String> {
        if let WorldKey::Name(_) = key {
            if let WorldItem::Interface { id, .. } = item {
                if self.interfaces[*id].name.is_some() {
                    return Some(self.id_of(*id).unwrap().into());
                }
            }
        }
        None
    }

    /// Returns the component model `external-id` value for the world import of
    /// `key` and `item`.
    ///
    /// See the component model explainer and 🏷️ for more information on this feature.
    pub fn external_id_value(&self, key: &WorldKey, item: &WorldItem) -> Option<String> {
        let _ = key;
        match item {
            WorldItem::Interface { external_id, .. } => external_id.clone(),
            WorldItem::Function(f) => f.external_id.clone(),
            WorldItem::Type { id, .. } => self.types[*id].external_id.clone(),
        }
    }

    /// Returns the interface that `id` uses a type from, if it uses a type from
    /// a different interface than `id` is defined within.
    ///
    /// If `id` is not a use-of-a-type or it's using a type in the same
    /// interface then `None` is returned.
    pub fn type_interface_dep(&self, id: TypeId) -> Option<InterfaceId> {
        let ty = &self.types[id];
        let dep = match ty.kind {
            TypeDefKind::Type(Type::Id(id)) => id,
            _ => return None,
        };
        let other = &self.types[dep];
        if ty.owner == other.owner {
            None
        } else {
            match other.owner {
                TypeOwner::Interface(id) => Some(id),
                _ => unreachable!(),
            }
        }
    }

    /// Returns an iterator of all interfaces that the interface `id` depends
    /// on.
    ///
    /// Interfaces may depend on others for type information to resolve type
    /// imports.
    ///
    /// Note that the returned iterator may yield the same interface as a
    /// dependency multiple times. Additionally only direct dependencies of `id`
    /// are yielded, not transitive dependencies.
    pub fn interface_direct_deps(&self, id: InterfaceId) -> impl Iterator<Item = InterfaceId> + '_ {
        self.interfaces[id]
            .types
            .iter()
            .filter_map(move |(_name, ty)| self.type_interface_dep(*ty))
    }

    /// Returns an iterator of all packages that the package `id` depends
    /// on.
    ///
    /// Packages may depend on others for type information to resolve type
    /// imports or interfaces to resolve worlds.
    ///
    /// Note that the returned iterator may yield the same package as a
    /// dependency multiple times. Additionally only direct dependencies of `id`
    /// are yielded, not transitive dependencies.
    pub fn package_direct_deps(&self, id: PackageId) -> impl Iterator<Item = PackageId> + '_ {
        let pkg = &self.packages[id];

        pkg.interfaces
            .iter()
            .flat_map(move |(_name, id)| self.interface_direct_deps(*id))
            .chain(pkg.worlds.iter().flat_map(move |(_name, id)| {
                let world = &self.worlds[*id];
                world
                    .imports
                    .iter()
                    .chain(world.exports.iter())
                    .filter_map(move |(_name, item)| match item {
                        WorldItem::Interface { id, .. } => Some(*id),
                        WorldItem::Function(_) => None,
                        WorldItem::Type { id, .. } => self.type_interface_dep(*id),
                    })
            }))
            .filter_map(move |iface_id| {
                let pkg = self.interfaces[iface_id].package?;
                if pkg == id { None } else { Some(pkg) }
            })
    }

    /// Returns a topological ordering of packages contained in this `Resolve`.
    ///
    /// This returns a list of `PackageId` such that when visited in order it's
    /// guaranteed that all dependencies will have been defined by prior items
    /// in the list.
    pub fn topological_packages(&self) -> Vec<PackageId> {
        let mut pushed = vec![false; self.packages.len()];
        let mut order = Vec::new();
        for (id, _) in self.packages.iter() {
            self.build_topological_package_ordering(id, &mut pushed, &mut order);
        }
        order
    }

    fn build_topological_package_ordering(
        &self,
        id: PackageId,
        pushed: &mut Vec<bool>,
        order: &mut Vec<PackageId>,
    ) {
        if pushed[id.index()] {
            return;
        }
        for dep in self.package_direct_deps(id) {
            self.build_topological_package_ordering(dep, pushed, order);
        }
        order.push(id);
        pushed[id.index()] = true;
    }

    #[doc(hidden)]
    pub fn assert_valid(&self) {
        let mut package_interfaces = Vec::new();
        let mut package_worlds = Vec::new();
        for (id, pkg) in self.packages.iter() {
            let mut interfaces = HashSet::new();
            for (name, iface) in pkg.interfaces.iter() {
                assert!(interfaces.insert(*iface));
                let iface = &self.interfaces[*iface];
                assert_eq!(name, iface.name.as_ref().unwrap());
                assert_eq!(iface.package.unwrap(), id);
            }
            package_interfaces.push(pkg.interfaces.values().copied().collect::<HashSet<_>>());
            let mut worlds = HashSet::new();
            for (name, world) in pkg.worlds.iter() {
                assert!(worlds.insert(*world));
                assert_eq!(
                    pkg.worlds.get_key_value(name),
                    Some((name, world)),
                    "`MutableKeys` impl may have been used to change a key's hash or equality"
                );
                let world = &self.worlds[*world];
                assert_eq!(*name, world.name);
                assert_eq!(world.package.unwrap(), id);
            }
            package_worlds.push(pkg.worlds.values().copied().collect::<HashSet<_>>());
        }

        let mut interface_types = Vec::new();
        for (id, iface) in self.interfaces.iter() {
            assert!(self.packages.get(iface.package.unwrap()).is_some());
            if iface.name.is_some() {
                match iface.clone_of {
                    Some(other) => {
                        assert_eq!(iface.name, self.interfaces[other].name);
                    }
                    None => {
                        assert!(package_interfaces[iface.package.unwrap().index()].contains(&id));
                    }
                }
            }

            for (name, ty) in iface.types.iter() {
                let ty = &self.types[*ty];
                assert_eq!(ty.name.as_ref(), Some(name));
                assert_eq!(ty.owner, TypeOwner::Interface(id));
            }
            interface_types.push(iface.types.values().copied().collect::<HashSet<_>>());
            for (name, f) in iface.functions.iter() {
                assert_eq!(*name, f.name);
            }
        }

        let mut world_types = Vec::new();
        for (id, world) in self.worlds.iter() {
            log::debug!("validating world {}", &world.name);
            if let Some(package) = world.package {
                assert!(self.packages.get(package).is_some());
                assert!(package_worlds[package.index()].contains(&id));
            }
            assert!(world.includes.is_empty());

            let mut types = HashSet::new();
            for (name, item) in world.imports.iter().chain(world.exports.iter()) {
                log::debug!("validating world item: {}", self.name_world_key(name));
                match item {
                    WorldItem::Interface { id, .. } => {
                        // Anonymous interfaces must belong to the same package,
                        // but interfaces through `implements` can be in any
                        // package.
                        if matches!(name, WorldKey::Name(_)) {
                            let iface = &self.interfaces[*id];
                            if iface.name.is_none() {
                                assert_eq!(iface.package, world.package);
                            }
                        }
                    }
                    WorldItem::Function(f) => {
                        assert!(!matches!(name, WorldKey::Interface(_)));
                        assert_eq!(f.name, name.clone().unwrap_name());
                    }
                    WorldItem::Type { id: ty, .. } => {
                        assert!(!matches!(name, WorldKey::Interface(_)));
                        assert!(types.insert(*ty));
                        let ty = &self.types[*ty];
                        assert_eq!(ty.name, Some(name.clone().unwrap_name()));
                        assert_eq!(ty.owner, TypeOwner::World(id));
                    }
                }
            }
            self.assert_world_elaborated(world);
            world_types.push(types);
        }

        for (ty_id, ty) in self.types.iter() {
            match ty.owner {
                TypeOwner::Interface(id) => {
                    assert!(self.interfaces.get(id).is_some());
                    assert!(interface_types[id.index()].contains(&ty_id));
                }
                TypeOwner::World(id) => {
                    assert!(self.worlds.get(id).is_some());
                    assert!(world_types[id.index()].contains(&ty_id));
                }
                TypeOwner::None => {}
            }
        }

        self.assert_topologically_sorted();
    }

    fn assert_topologically_sorted(&self) {
        let mut positions = IndexMap::default();
        for id in self.topological_packages() {
            let pkg = &self.packages[id];
            log::debug!("pkg {}", pkg.name);
            let prev = positions.insert(Some(id), IndexSet::default());
            assert!(prev.is_none());
        }
        positions.insert(None, IndexSet::default());

        for (id, iface) in self.interfaces.iter() {
            log::debug!("iface {:?}", iface.name);
            let ok = positions.get_mut(&iface.package).unwrap().insert(id);
            assert!(ok);
        }

        for (_, world) in self.worlds.iter() {
            log::debug!("world {:?}", world.name);

            let my_package = world.package;
            let my_package_pos = positions.get_index_of(&my_package).unwrap();

            for (_, item) in world.imports.iter().chain(&world.exports) {
                let id = match item {
                    WorldItem::Interface { id, .. } => *id,
                    _ => continue,
                };
                let other_package = self.interfaces[id].package;
                let other_package_pos = positions.get_index_of(&other_package).unwrap();

                assert!(other_package_pos <= my_package_pos);
            }
        }

        for (_id, ty) in self.types.iter() {
            log::debug!("type {:?} {:?}", ty.name, ty.owner);
            let other_id = match ty.kind {
                TypeDefKind::Type(Type::Id(ty)) => ty,
                _ => continue,
            };
            let other = &self.types[other_id];
            if ty.kind == other.kind {
                continue;
            }
            let my_interface = match ty.owner {
                TypeOwner::Interface(id) => id,
                _ => continue,
            };
            let other_interface = match other.owner {
                TypeOwner::Interface(id) => id,
                _ => continue,
            };

            let my_package = self.interfaces[my_interface].package;
            let other_package = self.interfaces[other_interface].package;
            let my_package_pos = positions.get_index_of(&my_package).unwrap();
            let other_package_pos = positions.get_index_of(&other_package).unwrap();

            assert!(other_package_pos <= my_package_pos);
        }
    }

    fn assert_world_elaborated(&self, world: &World) {
        for (key, item) in world.imports.iter() {
            log::debug!(
                "asserting elaborated world import {}",
                self.name_world_key(key)
            );
            match item {
                WorldItem::Type { id, .. } => self.assert_world_imports_type_deps(world, key, *id),

                // All types referred to must be imported.
                WorldItem::Function(f) => self.assert_world_function_imports_types(world, key, f),

                // All direct dependencies of this interface must be imported.
                WorldItem::Interface { id, .. } => {
                    for dep in self.interface_direct_deps(*id) {
                        assert!(
                            world.imports.contains_key(&WorldKey::Interface(dep)),
                            "world import of {} is missing transitive dep of {}",
                            self.name_world_key(key),
                            self.id_of(dep).unwrap(),
                        );
                    }
                }
            }
        }
        for (key, item) in world.exports.iter() {
            log::debug!(
                "asserting elaborated world export {}",
                self.name_world_key(key)
            );
            match item {
                // Types referred to by this function must be imported.
                WorldItem::Function(f) => self.assert_world_function_imports_types(world, key, f),

                // Dependencies of exported interfaces must also be exported, or
                // if imported then that entire chain of imports must be
                // imported and not exported.
                WorldItem::Interface { id, .. } => {
                    for dep in self.interface_direct_deps(*id) {
                        let dep_key = WorldKey::Interface(dep);
                        if world.exports.contains_key(&dep_key) {
                            continue;
                        }
                        self.foreach_interface_dep(dep, &mut |dep| {
                            let dep_key = WorldKey::Interface(dep);
                            assert!(
                                world.imports.contains_key(&dep_key),
                                "world should import {} (required by {})",
                                self.name_world_key(&dep_key),
                                self.name_world_key(key),
                            );
                            assert!(
                                !world.exports.contains_key(&dep_key),
                                "world should not export {} (required by {})",
                                self.name_world_key(&dep_key),
                                self.name_world_key(key),
                            );
                        });
                    }
                }

                // exported types not allowed at this time
                WorldItem::Type { .. } => unreachable!(),
            }
        }
    }

    fn assert_world_imports_type_deps(&self, world: &World, key: &WorldKey, ty: TypeId) {
        // If this is a `use` statement then the referred-to interface must be
        // imported into this world.
        let ty = &self.types[ty];
        if let TypeDefKind::Type(Type::Id(other)) = ty.kind {
            if let TypeOwner::Interface(id) = self.types[other].owner {
                let key = WorldKey::Interface(id);
                assert!(world.imports.contains_key(&key));
                return;
            }
        }

        // ... otherwise any named type that this type refers to, one level
        // deep, must be imported into this world under that name.

        let mut visitor = MyVisit(self, Vec::new());
        visitor.visit_type_def(self, ty);
        for ty in visitor.1 {
            let ty = &self.types[ty];
            let Some(name) = ty.name.clone() else {
                continue;
            };
            let dep_key = WorldKey::Name(name);
            assert!(
                world.imports.contains_key(&dep_key),
                "world import `{}` should also force an import of `{}`",
                self.name_world_key(key),
                self.name_world_key(&dep_key),
            );
        }

        struct MyVisit<'a>(&'a Resolve, Vec<TypeId>);

        impl TypeIdVisitor for MyVisit<'_> {
            fn before_visit_type_id(&mut self, id: TypeId) -> bool {
                self.1.push(id);
                // recurse into unnamed types to look at all named types
                self.0.types[id].name.is_none()
            }
        }
    }

    /// This asserts that all types referred to by `func` are imported into
    /// `world` under `WorldKey::Name`. Note that this is only applicable to
    /// named type
    fn assert_world_function_imports_types(&self, world: &World, key: &WorldKey, func: &Function) {
        for ty in func
            .parameter_and_result_types()
            .chain(func.kind.resource().map(Type::Id))
        {
            let Type::Id(id) = ty else {
                continue;
            };
            self.assert_world_imports_type_deps(world, key, id);
        }
    }

    /// Returns whether the `stability` annotation contained within `pkg_id`
    /// should be included or not.
    ///
    /// The `span` provided here is an optional span pointing to the item that
    /// is annotated with `stability`.
    ///
    /// Returns `Ok(true)` if the item is included, or `Ok(false)` if the item
    /// is not.
    ///
    /// # Errors
    ///
    /// Returns an error if the `pkg_id` isn't annotated with sufficient version
    /// information to have a `stability` annotation. For example if `pkg_id`
    /// has no version listed then an error will be returned if `stability`
    /// mentions a version.
    fn include_stability(
        &self,
        stability: &Stability,
        pkg_id: &PackageId,
        span: Span,
    ) -> ResolveResult<bool> {
        Ok(match stability {
            Stability::Unknown => true,
            // NOTE: deprecations are intentionally omitted -- an existing
            // `@since` takes precedence over `@deprecated`
            Stability::Stable { since, .. } => {
                let Some(p) = self.packages.get(*pkg_id) else {
                    // We can't check much without a package (possibly dealing
                    // with an item in an `UnresolvedPackage`), @since version &
                    // deprecations can't be checked because there's no package
                    // version to compare to.
                    //
                    // Feature requirements on stabilized features are ignored
                    // in resolved packages, so we do the same here.
                    return Ok(true);
                };

                // Use of feature gating with version specifiers inside a
                // package that is not versioned is not allowed
                let package_version = p.name.version.as_ref().ok_or_else(|| {
                    ResolveError::new_semantic(
                        span,
                        format!(
                            "package [{}] contains a feature gate with a version \
                         specifier, so it must have a version",
                            p.name
                        ),
                    )
                })?;

                // If the version on the feature gate is:
                // - released, then we can include it
                // - unreleased, then we must check the feature (if present)
                if since > package_version {
                    return Err(ResolveError::new_semantic(
                        span,
                        format!(
                            "feature gate cannot reference unreleased version \
                        {since} of package [{}] (current version {package_version})",
                            p.name
                        ),
                    ));
                }

                true
            }
            Stability::Unstable { feature, .. } => {
                self.features.contains(feature) || self.all_features
            }
        })
    }

    /// Performs the "elaboration process" necessary for the `world_id`
    /// specified to ensure that all of its transitive imports are listed.
    ///
    /// This function will take the unordered lists of the specified world's
    /// imports and exports and "elaborate" them to ensure that they're
    /// topographically sorted where all transitively required interfaces by
    /// imports, or exports, are listed. This will additionally validate that
    /// the exports are all valid and present, specifically with the restriction
    /// noted on `elaborate_world_exports`.
    ///
    /// The world is mutated in-place in this `Resolve`.
    fn elaborate_world(&mut self, world_id: WorldId, span: Span) -> ResolveResult<()> {
        // First process all imports. This is easier than exports since the only
        // requirement here is that all interfaces need to be added with a
        // topological order between them.
        let mut new_imports = IndexMap::default();
        let world = &self.worlds[world_id];

        // Sort the imports by "class" to ensure that this matches the order
        // that items are printed and that items are in topological order.
        //
        // When printing worlds in WIT:
        //
        // * interfaces come first
        // * types are next
        //   * type imports are first
        //   * type definitions are next
        //   * resource definitions have methods printed inline
        // * freestanding functions are last
        //
        // This reflects the topological order between items where types
        // can refer to imports and functions can refer to these types. Ordering
        // within a single class (e.g. imports depending on each other, types
        // referring to each other) is already preserved by other passes in this
        // file and general AST resolution. That means that a stable sort here
        // can be used to ensure that each class is in the right location
        // relative to the others.
        //
        // Overall this ensures that round-trips of WIT through wasm should
        // always produce the same result.
        let sort_key = |resolve: &Resolve, item: &WorldItem| match item {
            WorldItem::Interface { .. } => 0,
            WorldItem::Type { id, .. } => {
                let ty = &resolve.types[*id];
                match ty.kind {
                    TypeDefKind::Type(Type::Id(t)) if resolve.types[t].owner != ty.owner => 1,
                    _ => 2,
                }
            }
            WorldItem::Function(f) => {
                if f.kind.resource().is_none() {
                    3
                } else {
                    4
                }
            }
        };

        // Sort world items when we start to elaborate the world to start with a
        // topological view of items.
        let mut world_imports = world.imports.iter().collect::<Vec<_>>();
        world_imports.sort_by_key(|(_name, import)| sort_key(self, import));
        for (name, item) in world_imports {
            match item {
                // Interfaces get their dependencies added first followed by the
                // interface itself.
                WorldItem::Interface {
                    id,
                    stability,
                    docs,
                    external_id,
                    ..
                } => {
                    self.elaborate_world_import(
                        &mut new_imports,
                        name.clone(),
                        *id,
                        &stability,
                        docs,
                        external_id.as_deref(),
                    );
                }

                // Functions are added as-is since their dependence on types in
                // the world should already be satisfied.
                WorldItem::Function(_) => {
                    let prev = new_imports.insert(name.clone(), item.clone());
                    assert!(prev.is_none());
                }

                // Types may depend on an interface, in which case a (possibly)
                // recursive addition of that interface happens here. Afterwards
                // the type itself can be added safely.
                WorldItem::Type { id, .. } => {
                    if let Some(dep) = self.type_interface_dep(*id) {
                        self.elaborate_world_import(
                            &mut new_imports,
                            WorldKey::Interface(dep),
                            dep,
                            &self.types[*id].stability,
                            &Docs::default(),
                            None,
                        );
                    }
                    let prev = new_imports.insert(name.clone(), item.clone());
                    assert!(prev.is_none());
                }
            }
        }

        // Exports are trickier than imports, notably to uphold the invariant
        // required by `elaborate_world_exports`. To do this the exports are
        // partitioned into interfaces/functions. All functions are added to
        // the new exports list during this loop but interfaces are all deferred
        // to be handled in the `elaborate_world_exports` function.
        let mut new_exports = IndexMap::default();
        let mut export_interfaces = IndexMap::default();
        for (name, item) in world.exports.iter() {
            match item {
                WorldItem::Interface { .. } => {
                    let prev = export_interfaces.insert(name.clone(), item.clone());
                    assert!(prev.is_none());
                }
                WorldItem::Function(_) => {
                    let prev = new_exports.insert(name.clone(), item.clone());
                    assert!(prev.is_none());
                }
                WorldItem::Type { .. } => unreachable!(),
            }
        }

        self.elaborate_world_exports(&export_interfaces, &mut new_imports, &mut new_exports, span)?;

        // In addition to sorting at the start of elaboration also sort here at
        // the end of elaboration to handle types being interspersed with
        // interfaces as they're found.
        new_imports.sort_by_cached_key(|_name, import| sort_key(self, import));

        // And with all that done the world is updated in-place with
        // imports/exports.
        log::trace!("imports = {new_imports:?}");
        log::trace!("exports = {new_exports:?}");
        let world = &mut self.worlds[world_id];
        world.imports = new_imports;
        world.exports = new_exports;

        Ok(())
    }

    fn elaborate_world_import(
        &self,
        imports: &mut IndexMap<WorldKey, WorldItem>,
        key: WorldKey,
        id: InterfaceId,
        stability: &Stability,
        docs: &Docs,
        external_id: Option<&str>,
    ) {
        if imports.contains_key(&key) {
            return;
        }
        // Synthesized dependency imports carry no statement-level docs of their
        // own.
        for dep in self.interface_direct_deps(id) {
            self.elaborate_world_import(
                imports,
                WorldKey::Interface(dep),
                dep,
                stability,
                &Docs::default(),
                None,
            );
        }
        let prev = imports.insert(
            key,
            WorldItem::Interface {
                id,
                stability: stability.clone(),
                docs: docs.clone(),
                span: Default::default(),
                external_id: external_id.map(|s| s.to_string()),
            },
        );
        assert!(prev.is_none());
    }

    /// This function adds all of the interfaces in `export_interfaces` to the
    /// list of exports of the `world` specified.
    ///
    /// This method is more involved than adding imports because it is fallible.
    /// Chiefly what can happen is that the dependencies of all exports must be
    /// satisfied by other exports or imports, but not both. For example given a
    /// situation such as:
    ///
    /// ```wit
    /// interface a {
    ///     type t = u32
    /// }
    /// interface b {
    ///     use a.{t}
    /// }
    /// interface c {
    ///     use a.{t}
    ///     use b.{t as t2}
    /// }
    /// ```
    ///
    /// where `c` depends on `b` and `a` where `b` depends on `a`, then the
    /// purpose of this method is to reject this world:
    ///
    /// ```wit
    /// world foo {
    ///     export a
    ///     export c
    /// }
    /// ```
    ///
    /// The reasoning here is unfortunately subtle and is additionally the
    /// subject of WebAssembly/component-model#208. Effectively the `c`
    /// interface depends on `b`, but it's not listed explicitly as an import,
    /// so it's then implicitly added as an import. This then transitively
    /// depends on `a` so it's also added as an import. At this point though `c`
    /// also depends on `a`, and it's also exported, so naively it should depend
    /// on the export and not implicitly add an import. This means though that
    /// `c` has access to two copies of `a`, one imported and one exported. This
    /// is not valid, especially in the face of resource types.
    ///
    /// Overall this method is tasked with rejecting the above world by walking
    /// over all the exports and adding their dependencies. Each dependency is
    /// recorded with whether it's required to be imported, and then if an
    /// export is added for something that's required to be an error then the
    /// operation fails.
    fn elaborate_world_exports(
        &self,
        export_interfaces: &IndexMap<WorldKey, WorldItem>,
        imports: &mut IndexMap<WorldKey, WorldItem>,
        exports: &mut IndexMap<WorldKey, WorldItem>,
        span: Span,
    ) -> ResolveResult<()> {
        let mut required_imports = HashSet::new();
        for (key, item) in export_interfaces.iter() {
            let name = self.name_world_key(&key);
            let ok = add_world_export(
                self,
                imports,
                exports,
                &export_interfaces,
                &mut required_imports,
                key.clone(),
                item.clone(),
                true,
            );
            if !ok {
                // FIXME: this is not a great error message and basically no
                // one will know what to do when it gets printed. Improving
                // this error message, however, is a chunk of work that may
                // not be best spent doing this at this time, so I'm writing
                // this comment instead.
                //
                // More-or-less what should happen here is that a "path"
                // from this interface to the conflicting interface should
                // be printed. It should be explained why an import is being
                // injected, why that's conflicting with an export, and
                // ideally with a suggestion of "add this interface to the
                // export list to fix this error".
                //
                // That's a lot of info that's not easy to get at without
                // more refactoring, so it's left to a future date in the
                // hopes that most folks won't actually run into this for
                // the time being.
                return Err(ResolveError::from(
                    ResolveErrorKind::InvalidTransitiveDependency { name, span },
                ));
            }
        }
        return Ok(());

        fn add_world_export(
            resolve: &Resolve,
            imports: &mut IndexMap<WorldKey, WorldItem>,
            exports: &mut IndexMap<WorldKey, WorldItem>,
            export_interfaces: &IndexMap<WorldKey, WorldItem>,
            required_imports: &mut HashSet<InterfaceId>,
            key: WorldKey,
            item: WorldItem,
            add_export: bool,
        ) -> bool {
            if exports.contains_key(&key) {
                if add_export {
                    return true;
                } else {
                    return false;
                }
            }
            let (id, stability, external_id) = match &item {
                WorldItem::Interface {
                    id,
                    stability,
                    external_id,
                    ..
                } => (*id, stability, external_id),
                _ => unreachable!(),
            };
            // If this is an import and it's already in the `imports` set then
            // we can skip it as we've already visited this interface.
            if !add_export && required_imports.contains(&id) {
                return true;
            }
            let ok = resolve.interface_direct_deps(id).all(|dep| {
                let item = WorldItem::Interface {
                    id: dep,
                    stability: stability.clone(),
                    docs: Default::default(),
                    span: Default::default(),
                    external_id: external_id.clone(),
                };
                let key = WorldKey::Interface(dep);
                let add_export = add_export && export_interfaces.contains_key(&key);
                add_world_export(
                    resolve,
                    imports,
                    exports,
                    export_interfaces,
                    required_imports,
                    key,
                    item,
                    add_export,
                )
            });
            if !ok {
                return false;
            }
            if add_export {
                if required_imports.contains(&id) {
                    return false;
                }
                let prev = exports.insert(key.clone(), item);
                assert!(prev.is_none());
            } else {
                required_imports.insert(id);
                if !imports.contains_key(&key) {
                    imports.insert(key.clone(), item);
                }
            }
            true
        }
    }

    /// Remove duplicate imports from a world if they import from the same
    /// interface with semver-compatible versions.
    ///
    /// This will merge duplicate interfaces present at multiple versions in
    /// both a world by selecting the larger version of the two interfaces. This
    /// requires that the interfaces are indeed semver-compatible and it means
    /// that some imports might be removed and replaced. Note that this is only
    /// done within a single semver track, for example the world imports 0.2.0
    /// and 0.2.1 then the result afterwards will be that it imports
    /// 0.2.1. If, however, 0.3.0 where imported then the final result would
    /// import both 0.2.0 and 0.3.0.
    pub fn merge_world_imports_based_on_semver(&mut self, world_id: WorldId) -> anyhow::Result<()> {
        let world = &self.worlds[world_id];

        // The first pass here is to build a map of "semver tracks" where they
        // key is per-interface and the value is the maximal version found in
        // that semver-compatible-track plus the interface which is the maximal
        // version.
        //
        // At the same time a `to_remove` set is maintained to remember what
        // interfaces are being removed from `from` and `into`. All of
        // `to_remove` are placed with a known other version.
        let mut semver_tracks = HashMap::new();
        let mut to_remove = HashSet::new();
        for (key, _) in world.imports.iter() {
            let iface_id = match key {
                WorldKey::Interface(id) => *id,
                WorldKey::Name(_) => continue,
            };
            let (track, version) = match self.semver_track(iface_id) {
                Some(track) => track,
                None => continue,
            };
            log::debug!(
                "{} is on track {}/{}",
                self.id_of(iface_id).unwrap(),
                track.0,
                track.1,
            );
            match semver_tracks.entry(track.clone()) {
                Entry::Vacant(e) => {
                    e.insert((version, iface_id));
                }
                Entry::Occupied(mut e) => match version.cmp(&e.get().0) {
                    Ordering::Greater => {
                        to_remove.insert(e.get().1);
                        e.insert((version, iface_id));
                    }
                    Ordering::Equal => {}
                    Ordering::Less => {
                        to_remove.insert(iface_id);
                    }
                },
            }
        }

        // Build a map of "this interface is replaced with this interface" using
        // the results of the loop above.
        let mut replacements = HashMap::new();
        for id in to_remove {
            let (track, _) = self.semver_track(id).unwrap();
            let (_, latest) = semver_tracks[&track];
            let prev = replacements.insert(id, latest);
            assert!(prev.is_none());
        }
        // Explicit drop needed for hashbrown compatibility - hashbrown's HashMap
        // destructor may access stored references, extending the borrow.
        drop(semver_tracks);

        // Validate that `merge_world_item` succeeds for merging all removed
        // interfaces with their replacement. This is a double-check that the
        // semver version is actually correct and all items present in the old
        // interface are in the new.
        for (to_replace, replace_with) in replacements.iter() {
            self.merge_world_item(
                &WorldItem::Interface {
                    id: *to_replace,
                    stability: Default::default(),
                    docs: Default::default(),
                    span: Default::default(),
                    external_id: Default::default(),
                },
                &WorldItem::Interface {
                    id: *replace_with,
                    stability: Default::default(),
                    docs: Default::default(),
                    span: Default::default(),
                    external_id: Default::default(),
                },
            )
            .with_context(|| {
                let old_name = self.id_of(*to_replace).unwrap();
                let new_name = self.id_of(*replace_with).unwrap();
                format!(
                    "failed to upgrade `{old_name}` to `{new_name}`, was \
                     this semver-compatible update not semver compatible?"
                )
            })?;
        }

        for (to_replace, replace_with) in replacements.iter() {
            log::debug!(
                "REPLACE {} => {}",
                self.id_of(*to_replace).unwrap(),
                self.id_of(*replace_with).unwrap(),
            );
        }

        // Finally perform the actual transformation of the imports/exports.
        // Here all imports are removed if they're replaced and otherwise all
        // imports have their dependencies updated, possibly transitively, to
        // point to the new interfaces in `replacements`.
        //
        // Afterwards exports are additionally updated, but only their
        // dependencies on imports which were remapped. Exports themselves are
        // not deduplicated and/or removed.
        for (key, item) in mem::take(&mut self.worlds[world_id].imports) {
            if let WorldItem::Interface { id, .. } = item {
                if replacements.contains_key(&id) {
                    continue;
                }
            }

            self.update_interface_deps_of_world_item(&item, &replacements);

            let prev = self.worlds[world_id].imports.insert(key, item);
            assert!(prev.is_none());
        }
        for (key, item) in mem::take(&mut self.worlds[world_id].exports) {
            self.update_interface_deps_of_world_item(&item, &replacements);
            let prev = self.worlds[world_id].exports.insert(key, item);
            assert!(prev.is_none());
        }

        // Run through `elaborate_world` to reorder imports as appropriate and
        // fill anything back in if it's actually required by exports. For now
        // this doesn't tamper with exports at all. Also note that this is
        // applied to all worlds in this `Resolve` because interfaces were
        // modified directly.
        let ids = self.worlds.iter().map(|(id, _)| id).collect::<Vec<_>>();
        for world_id in ids {
            let world_span = self.worlds[world_id].span;
            self.elaborate_world(world_id, world_span)?;
        }

        #[cfg(debug_assertions)]
        self.assert_valid();

        Ok(())
    }

    fn update_interface_deps_of_world_item(
        &mut self,
        item: &WorldItem,
        replacements: &HashMap<InterfaceId, InterfaceId>,
    ) {
        match *item {
            WorldItem::Type { id, .. } => self.update_interface_dep_of_type(id, &replacements),
            WorldItem::Interface { id, .. } => {
                let types = self.interfaces[id]
                    .types
                    .values()
                    .copied()
                    .collect::<Vec<_>>();
                for ty in types {
                    self.update_interface_dep_of_type(ty, &replacements);
                }
            }
            WorldItem::Function(_) => {}
        }
    }

    /// Returns the "semver track" of an interface plus the interface's version.
    ///
    /// This function returns `None` if the interface `id` has a package without
    /// a version. If the version is present, however, the first element of the
    /// tuple returned is a "semver track" for the specific interface. The
    /// version listed in `PackageName` will be modified so all
    /// semver-compatible versions are listed the same way.
    ///
    /// The second element in the returned tuple is this interface's package's
    /// version.
    fn semver_track(&self, id: InterfaceId) -> Option<((PackageName, String), &Version)> {
        let iface = &self.interfaces[id];
        let pkg = &self.packages[iface.package?];
        let version = pkg.name.version.as_ref()?;
        let mut name = pkg.name.clone();
        name.version = Some(PackageName::version_compat_track(version));
        Some(((name, iface.name.clone()?), version))
    }

    /// If `ty` is a definition where it's a `use` from another interface, then
    /// change what interface it's using from according to the pairs in the
    /// `replacements` map.
    fn update_interface_dep_of_type(
        &mut self,
        ty: TypeId,
        replacements: &HashMap<InterfaceId, InterfaceId>,
    ) {
        let to_replace = match self.type_interface_dep(ty) {
            Some(id) => id,
            None => return,
        };
        let replace_with = match replacements.get(&to_replace) {
            Some(id) => id,
            None => return,
        };
        let dep = match self.types[ty].kind {
            TypeDefKind::Type(Type::Id(id)) => id,
            _ => return,
        };
        let name = self.types[dep].name.as_ref().unwrap();
        // Note the infallible name indexing happening here. This should be
        // previously validated with `merge_world_item` to succeed.
        let replacement_id = self.interfaces[*replace_with].types[name];
        self.types[ty].kind = TypeDefKind::Type(Type::Id(replacement_id));
    }

    /// Returns the core wasm module/field names for the specified `import`.
    ///
    /// This function will return the core wasm module/field that can be used to
    /// use `import` with the name `mangling` scheme specified as well. This can
    /// be useful for bindings generators, for example, and these names are
    /// recognized by `wit-component` and `wasm-tools component new`.
    pub fn wasm_import_name(
        &self,
        mangling: ManglingAndAbi,
        import: WasmImport<'_>,
    ) -> (String, String) {
        match mangling {
            ManglingAndAbi::Standard32 => match import {
                WasmImport::Func { interface, func } => {
                    let module = match interface {
                        Some(key) => format!("cm32p2|{}", self.name_canonicalized_world_key(key)),
                        None => format!("cm32p2"),
                    };
                    (module, func.name.clone())
                }
                WasmImport::ResourceIntrinsic {
                    interface,
                    resource,
                    intrinsic,
                } => {
                    let name = self.types[resource].name.as_ref().unwrap();
                    let (prefix, name) = match intrinsic {
                        ResourceIntrinsic::ImportedDrop => ("", format!("{name}_drop")),
                        ResourceIntrinsic::ExportedDrop => ("_ex_", format!("{name}_drop")),
                        ResourceIntrinsic::ExportedNew => ("_ex_", format!("{name}_new")),
                        ResourceIntrinsic::ExportedRep => ("_ex_", format!("{name}_rep")),
                    };
                    let module = match interface {
                        Some(key) => {
                            format!("cm32p2|{prefix}{}", self.name_canonicalized_world_key(key))
                        }
                        None => {
                            assert_eq!(prefix, "");
                            format!("cm32p2")
                        }
                    };
                    (module, name)
                }
                WasmImport::FutureIntrinsic { .. } | WasmImport::StreamIntrinsic { .. } => {
                    panic!(
                        "at the time of writing, standard32 name mangling only supports the \
                         synchronous ABI and does not define future/stream intrinsic imports; \
                         use legacy mangling for these imports"
                    )
                }
            },
            ManglingAndAbi::Legacy(abi) => match import {
                WasmImport::Func { interface, func } => {
                    let module = match interface {
                        Some(key) => self.name_world_key(key),
                        None => format!("$root"),
                    };
                    (module, format!("{}{}", abi.import_prefix(), func.name))
                }
                WasmImport::ResourceIntrinsic {
                    interface,
                    resource,
                    intrinsic,
                } => {
                    let name = self.types[resource].name.as_ref().unwrap();
                    let (prefix, name) = match intrinsic {
                        ResourceIntrinsic::ImportedDrop => ("", format!("[resource-drop]{name}")),
                        ResourceIntrinsic::ExportedDrop => {
                            ("[export]", format!("[resource-drop]{name}"))
                        }
                        ResourceIntrinsic::ExportedNew => {
                            ("[export]", format!("[resource-new]{name}"))
                        }
                        ResourceIntrinsic::ExportedRep => {
                            ("[export]", format!("[resource-rep]{name}"))
                        }
                    };
                    let module = match interface {
                        Some(key) => format!("{prefix}{}", self.name_world_key(key)),
                        None => {
                            assert_eq!(prefix, "");
                            format!("$root")
                        }
                    };
                    (module, format!("{}{name}", abi.import_prefix()))
                }
                WasmImport::FutureIntrinsic {
                    interface,
                    func,
                    ty,
                    intrinsic,
                    exported,
                    async_,
                } => {
                    let module_prefix = if exported { "[export]" } else { "" };
                    let module = match interface {
                        Some(key) => format!("{module_prefix}{}", self.name_world_key(key)),
                        None => format!("{module_prefix}$root"),
                    };
                    let type_index = match ty {
                        Some(ty) => func
                            .find_futures_and_streams(self)
                            .into_iter()
                            .position(|candidate| candidate == ty)
                            .unwrap_or_else(|| {
                                panic!(
                                    "future type {ty:?} not found in `find_futures_and_streams` for `{}`",
                                    func.name
                                )
                            })
                            .to_string(),
                        None => "unit".to_string(),
                    };
                    let (async_prefix, name) = match intrinsic {
                        FutureIntrinsic::New => {
                            assert!(!async_, "future.new cannot be async-lowered");
                            ("", "new")
                        }
                        FutureIntrinsic::Read => {
                            (if async_ { "[async-lower]" } else { "" }, "read")
                        }
                        FutureIntrinsic::Write => {
                            (if async_ { "[async-lower]" } else { "" }, "write")
                        }
                        FutureIntrinsic::CancelRead => {
                            (if async_ { "[async-lower]" } else { "" }, "cancel-read")
                        }
                        FutureIntrinsic::CancelWrite => {
                            (if async_ { "[async-lower]" } else { "" }, "cancel-write")
                        }
                        FutureIntrinsic::DropReadable => {
                            assert!(!async_, "future.drop-readable cannot be async-lowered");
                            ("", "drop-readable")
                        }
                        FutureIntrinsic::DropWritable => {
                            assert!(!async_, "future.drop-writable cannot be async-lowered");
                            ("", "drop-writable")
                        }
                    };
                    (
                        module,
                        format!("{async_prefix}[future-{name}-{type_index}]{}", func.name),
                    )
                }
                WasmImport::StreamIntrinsic {
                    interface,
                    func,
                    ty,
                    intrinsic,
                    exported,
                    async_,
                } => {
                    let module_prefix = if exported { "[export]" } else { "" };
                    let module = match interface {
                        Some(key) => format!("{module_prefix}{}", self.name_world_key(key)),
                        None => format!("{module_prefix}$root"),
                    };
                    let type_index = match ty {
                        Some(ty) => func
                            .find_futures_and_streams(self)
                            .into_iter()
                            .position(|candidate| candidate == ty)
                            .unwrap_or_else(|| {
                                panic!(
                                    "stream type {ty:?} not found in `find_futures_and_streams` for `{}`",
                                    func.name
                                )
                            })
                            .to_string(),
                        None => "unit".to_string(),
                    };
                    let (async_prefix, name) = match intrinsic {
                        StreamIntrinsic::New => {
                            assert!(!async_, "stream.new cannot be async-lowered");
                            ("", "new")
                        }
                        StreamIntrinsic::Read => {
                            (if async_ { "[async-lower]" } else { "" }, "read")
                        }
                        StreamIntrinsic::Write => {
                            (if async_ { "[async-lower]" } else { "" }, "write")
                        }
                        StreamIntrinsic::CancelRead => {
                            (if async_ { "[async-lower]" } else { "" }, "cancel-read")
                        }
                        StreamIntrinsic::CancelWrite => {
                            (if async_ { "[async-lower]" } else { "" }, "cancel-write")
                        }
                        StreamIntrinsic::DropReadable => {
                            assert!(!async_, "stream.drop-readable cannot be async-lowered");
                            ("", "drop-readable")
                        }
                        StreamIntrinsic::DropWritable => {
                            assert!(!async_, "stream.drop-writable cannot be async-lowered");
                            ("", "drop-writable")
                        }
                    };
                    (
                        module,
                        format!("{async_prefix}[stream-{name}-{type_index}]{}", func.name),
                    )
                }
            },
        }
    }

    /// Returns the core wasm export name for the specified `export`.
    ///
    /// This is the same as [`Resolve::wasm_import_name`], except for exports.
    pub fn wasm_export_name(&self, mangling: ManglingAndAbi, export: WasmExport<'_>) -> String {
        match mangling {
            ManglingAndAbi::Standard32 => match export {
                WasmExport::Func {
                    interface,
                    func,
                    kind,
                } => {
                    let mut name = String::from("cm32p2|");
                    if let Some(interface) = interface {
                        let s = self.name_canonicalized_world_key(interface);
                        name.push_str(&s);
                    }
                    name.push_str("|");
                    name.push_str(&func.name);
                    match kind {
                        WasmExportKind::Normal => {}
                        WasmExportKind::PostReturn => name.push_str("_post"),
                        WasmExportKind::Callback => todo!(
                            "not yet supported: \
                             async callback functions using standard name mangling"
                        ),
                    }
                    name
                }
                WasmExport::ResourceDtor {
                    interface,
                    resource,
                } => {
                    let name = self.types[resource].name.as_ref().unwrap();
                    let interface = self.name_canonicalized_world_key(interface);
                    format!("cm32p2|{interface}|{name}_dtor")
                }
                WasmExport::Memory => "cm32p2_memory".to_string(),
                WasmExport::Initialize => "cm32p2_initialize".to_string(),
                WasmExport::Realloc => "cm32p2_realloc".to_string(),
            },
            ManglingAndAbi::Legacy(abi) => match export {
                WasmExport::Func {
                    interface,
                    func,
                    kind,
                } => {
                    let mut name = abi.export_prefix().to_string();
                    match kind {
                        WasmExportKind::Normal => {}
                        WasmExportKind::PostReturn => name.push_str("cabi_post_"),
                        WasmExportKind::Callback => {
                            assert!(matches!(abi, LiftLowerAbi::AsyncCallback));
                            name = format!("[callback]{name}")
                        }
                    }
                    if let Some(interface) = interface {
                        let s = self.name_world_key(interface);
                        name.push_str(&s);
                        name.push_str("#");
                    }
                    name.push_str(&func.name);
                    name
                }
                WasmExport::ResourceDtor {
                    interface,
                    resource,
                } => {
                    let name = self.types[resource].name.as_ref().unwrap();
                    let interface = self.name_world_key(interface);
                    format!("{}{interface}#[dtor]{name}", abi.export_prefix())
                }
                WasmExport::Memory => "memory".to_string(),
                WasmExport::Initialize => "_initialize".to_string(),
                WasmExport::Realloc => "cabi_realloc".to_string(),
            },
        }
    }

    /// This method will rewrite the `world` provided to ensure that, where
    /// necessary, all types in interfaces referred to by the `world` have
    /// nominal type ids for bindings generation.
    ///
    /// The need for this method primarily arises from bindings generators
    /// generating types in a programming language. Bindings generators try to
    /// generate a type-per-WIT-type but this becomes problematic in situations
    /// such as when an `interface` is both imported and exported. For example:
    ///
    /// ```wit
    /// interface x {
    ///    resource r;
    /// }
    ///
    /// world foo {
    ///     import x;
    ///     export x;
    /// }
    /// ```
    ///
    /// Here the `r` resource, before this method, exists once within this
    /// [`Resolve`]. This is a problem for bindings generators because guest
    /// languages typically want to represent this world with two types: one
    /// for the import and one for the export. This matches component model
    /// semantics where `r` is a different type between the import and the
    /// export.
    ///
    /// The purpose of this method is to ensure that languages with nominal
    /// types, where type identity is unique based on definition not structure,
    /// will have an easier time generating bindings. This method will
    /// duplicate the interface `x`, for example, and everything it contains.
    /// This means that the `world foo` above will have a different
    /// `InterfaceId` for the import and the export of `x`, despite them using
    /// the same interface in WIT. This is intended to make bindings generators'
    /// jobs much easier because now Id-uniqueness matches the semantic meaning
    /// of the world as well.
    ///
    /// This function will rewrite imported/exported interfaces, as appropriate,
    /// to all have unique ids if they would otherwise overlap with the imports.
    pub fn generate_nominal_type_ids(&mut self, world_id: WorldId) {
        let mut seen = HashSet::new();
        let mut interface_keys_rewritten = HashSet::new();
        let mut maps = CloneMaps::default();

        // Pull out the imports/exports from `self` to have simultaneous
        // borrows.
        let world = &mut self.worlds[world_id];
        let mut imports = mem::take(&mut world.imports);
        let mut exports = mem::take(&mut world.exports);

        // Notably visit `imports` first as they always get priority in the
        // order of having things imported from them. After `imports` are
        // visited then process all `exports`.
        log::trace!("nominalizing world imports");
        self.nominalize_world_items(
            &mut maps,
            world_id,
            &mut imports,
            &mut seen,
            &mut interface_keys_rewritten,
        );
        log::trace!("nominalizing world exports");
        self.nominalize_world_items(
            &mut maps,
            world_id,
            &mut exports,
            &mut seen,
            &mut interface_keys_rewritten,
        );

        // Put that thing back where it came from, or so help me!
        let world = &mut self.worlds[world_id];
        assert!(world.imports.is_empty());
        assert!(world.exports.is_empty());
        world.imports = imports;
        world.exports = exports;

        #[cfg(debug_assertions)]
        self.assert_valid();
    }

    /// Implementation of `generate_nominal_type_ids` which processes either a
    /// world's imports or exports as specified in `items`.
    ///
    /// The parameters here are:
    ///
    /// * `maps` - the set of types that have previously been cloned by the
    ///   previous stage, if any. This is used to update any dependencies of an
    ///   interface that is cloned.
    ///
    /// * `world` - the world that's being nominalized.
    ///
    /// * `items` - either `imports` or `exports` for the `world`. This is
    ///   mutated in-place to have keys/items rewritten as-needed.
    ///
    /// * `seen` - the set of interfaces that have been seen previously when
    ///   iterating over this world. All interfaces processed are added to this
    ///   set.
    ///
    /// * `interface_keys_rewritten` - a distinct set from `seen` which only
    ///   tracks rewritten interfaces which have `WorldKey::Interface`. This is
    ///   the set of interfaces which dependencies can pull types from, which
    ///   needs to be tracked separately to know when to rewrite.
    fn nominalize_world_items(
        &mut self,
        maps: &mut CloneMaps,
        world: WorldId,
        items: &mut IndexMap<WorldKey, WorldItem>,
        seen: &mut HashSet<InterfaceId>,
        interface_keys_rewritten: &mut HashSet<InterfaceId>,
    ) {
        // Overall the problem that this function is trying to solve is not an
        // easy one. The input is an AST-like structure and the goal of this
        // function is to effectively perform a name resolution pass. The end
        // result is that if a thing points to another thing (e.g. type-use or
        // interface id) then that represents the actual name resolution of what
        // it points to.
        //
        // Currently, though, this isn't really a full-blown name resolution
        // pass. This is a pretty simple "do things in the right order" and name
        // resolution pops out. The conventions of WIT and how it translates to
        // components is what falls out of this loop below.
        //
        // The first rule of WIT is that interfaces can only use types from
        // other interface imports/exports. This notably excludes named imports.
        // For example:
        //
        //      interface a {
        //          type t = u32;
        //      }
        //
        //      interface b {
        //          use a.{t};
        //      }
        //
        //      world w {
        //          import a;
        //          import b; // uses `import a`
        //          import c: b; // also uses `import a`
        //          import d: interface {
        //              use a.{t}; // uses `import a`
        //          }
        //      }
        //
        // The next rule is that exported interfaces will use types from
        // imports, unless the interface is also exported. For example:
        //
        //      interface a {
        //          type t = u32;
        //      }
        //
        //      interface b {
        //          use a.{t};
        //      }
        //
        //      world w1 {
        //          import a;
        //
        //          export b; // uses `import a`
        //          export c: b; // uses `import a`
        //          export d: interface {
        //              use a.{t}; // uses `import a`
        //          }
        //      }
        //
        //      world w2 {
        //          export a;
        //          export b; // uses `export a`
        //          export c: b; // uses `export a`
        //          export d: interface {
        //              use a.{t}; // uses `export a`
        //          }
        //      }
        //
        // Finally, named imports, such as `import a: b` and `import a:
        // interface { ... }` cannot be used by anything. They can only
        // reference types in other interface imports/exports.
        //
        // Overall this is a pretty simplistic system. It's "good enough" for
        // now but will almost certainly be expanded over time. The hope is that
        // expanding this involves making this function more complicated but
        // ideally nowhere else.

        // Given all that intro, the first thing we need to prioritize is that
        // `WorldKey::Name`'d interfaces are visited after `WorldKey::Interface`
        // interfaces. This is a bit of a weird result of how this function is
        // implemented right now. This visit order is a bit of a hack and
        // probably won't live beyond making WIT more powerful.
        //
        // Anyway, the reason for this has to do with the `CloneMaps` down
        // below. Basically what we're doing here is cloning interfaces, but
        // when doing so we need to be able to rewrite references to
        // previously-cloned interfaces if need be. `CloneMaps` represents the
        // aggregate results of all previous clones. Due to named interfaces
        // never being importable-from it means that mutations to `CloneMaps`
        // are discarded when named interfaces are cloned. The trick here
        // happens where this unconditionally preserves all modifications to
        // `CloneMaps` for `WorldKey::Interface` clones. Behaivor then "falls
        // out" where references to cloned interfaces are naturally rewritten.
        //
        // This all falls down, however, if an import is cloned and recorded.
        // Interfaces can be both exported and imported, which would mean that
        // the import and export are both cloned, and both need to be preserved
        // in `CloneMaps`. Right now `CloneMaps` requires uniqueness when
        // cloning (e.g. can't clone the same thing twice).
        //
        // Long story short: it's a hack that sort order here is the way it is.
        // Sorry. Be prepared to delete this should WIT get more powerful.
        let mut order = items.iter().enumerate().collect::<Vec<_>>();
        order.sort_by_key(|(_, (key, item))| match (key, item) {
            (WorldKey::Name(_), WorldItem::Interface { .. }) => 1,
            _ => 0,
        });
        let mut to_rewrite = IndexMap::default();
        for (i, (key, item)) in order {
            let id = match item {
                WorldItem::Interface { id, .. } => *id,

                // Functions/types aren't rewritten, they're all nominal at the
                // world-level anyway.
                WorldItem::Function(_) | WorldItem::Type { .. } => continue,
            };

            // If this interface itself is being visited for the second time
            // (e.g. imported & exported, imported twice, exported twice, etc),
            // or if any dependency of this interface is rewritten, then the
            // interface itself needs to be rewritten. Otherwise continue
            // onwards.
            let duplicated = !seen.insert(id);
            let any_dep_rewritten = self
                .interface_direct_deps(id)
                .any(|dep| interface_keys_rewritten.contains(&dep));
            if !(duplicated || any_dep_rewritten) {
                log::trace!("{} already nominal", self.name_world_key(key));
                continue;
            }
            log::trace!("{} getting rewritten nominal", self.name_world_key(key));

            // If this is `WorldKey::Interface` then register this in the
            // `interface_keys_rewritten` map, and also plumb this through to
            // the rewriting stage to know whether `maps` needs to be reset or
            // not.
            let is_name = matches!(key, WorldKey::Name(_));
            if !is_name {
                assert!(interface_keys_rewritten.insert(id));
            }

            to_rewrite
                .entry(id)
                .or_insert(Vec::new())
                .push((i, is_name));
        }

        // Now that we know what to rewrite, rewrite everything.
        //
        // The trickiest part here is deciding what to do with `maps`. As
        // interfaces are cloned they'll record all remappings of
        // types/interfaces/etc within `maps`. We don't want to persist
        // everything because if an interface is cloned twice then everything
        // will get overwritten/corrupted within the map. In theory what we want
        // is for `maps` to track, for any one interface, just the transitive
        // set of dependencies for that interface and how they've been cloned.
        // What's implemented here is an approximation of this that should work
        // for now.
        //
        // Notably `to_rewrite` is an ordered list keyed by `InterfaceId`. This
        // means that if we walk `to_rewrite` in order we're walking this in
        // topological order. Second we then additionally sort the `list` for
        // each `to_rewrite` entry to ensure that all `WorldKey::Name` items are
        // visited first. In doing so we also discard all mutations to `maps`
        // after visiting is done. In effect what this does is it discards all
        // modifications due to `WorldKey::Name`, because nothing can depend on
        // those interfaces, and then it preserves modifications for
        // `WorldKey::Interface`, which other interfaces can indeed depend on.
        // In the end this basically does a very careful walk over a very
        // careful organization of `to_rewrite`.
        //
        // This'll need massive refactoring if WIT gets the ability to express
        // arbitrary edges between interfaces. It's WIT-level restriction right
        // now of sorts. There's no current way to model a `import a: i;` where
        // `i`'s dependencies are pulled from `import b: dep`, for example.
        // Right now if `i` depends on `dep` then that just always results in
        // `import dep`.
        for (id, mut list) in to_rewrite {
            list.sort_by_key(|(_, is_name)| if *is_name { 0 } else { 1 });
            for (i, is_name) in list {
                let prev_maps = if is_name { Some(maps.clone()) } else { None };

                let mut cloner = clone::Cloner::new(
                    self,
                    maps,
                    TypeOwner::World(world),
                    TypeOwner::World(world),
                );

                let mut new_id = id;
                cloner.new_package = cloner.resolve.interfaces[id].package;
                cloner.interface(&mut new_id);
                let (key, prev) = items.get_index_mut(i).unwrap();
                match prev {
                    WorldItem::Interface { id, .. } => *id = new_id,
                    _ => unreachable!(),
                }

                match key {
                    // If the key for this is an `Interface` then that means we
                    // need to update the key as well. Here that's replaced by-index
                    // in the `IndexMap` to preserve the same ordering as before,
                    // and this operation should always succeed since `new_id` is
                    // fresh, hence the `unwrap()`.
                    WorldKey::Interface(_) => {
                        items.replace_index(i, WorldKey::Interface(new_id)).unwrap();
                    }

                    // Name-based keys don't need updating as they only contain a
                    // string, no ids.
                    WorldKey::Name(_) => {}
                }

                if let Some(prev) = prev_maps {
                    *maps = prev;
                }
            }
        }
    }
}

/// Possible imports that can be passed to [`Resolve::wasm_import_name`].
#[derive(Debug)]
pub enum WasmImport<'a> {
    /// A WIT function is being imported. Optionally from an interface.
    Func {
        /// The name of the interface that the function is being imported from.
        ///
        /// If the function is imported directly from the world then this is
        /// `None`.
        interface: Option<&'a WorldKey>,

        /// The function being imported.
        func: &'a Function,
    },

    /// A resource-related intrinsic is being imported.
    ResourceIntrinsic {
        /// The optional interface to import from, same as `WasmImport::Func`.
        interface: Option<&'a WorldKey>,

        /// The resource that's being operated on.
        resource: TypeId,

        /// The intrinsic that's being imported.
        intrinsic: ResourceIntrinsic,
    },

    /// A future-related intrinsic is being imported.
    FutureIntrinsic {
        /// The optional interface to import from, same as `WasmImport::Func`.
        interface: Option<&'a WorldKey>,

        /// The function whose signature this future type appears in.
        func: &'a Function,

        /// The future type appearing in `func.find_futures_and_streams(resolve)`.
        ///
        /// Use `None` for the special `unit` payload case.
        ty: Option<TypeId>,

        /// The intrinsic that's being imported.
        intrinsic: FutureIntrinsic,

        /// Whether this import is for an exported WIT function.
        ///
        /// This controls whether the module name is prefixed with `[export]`.
        exported: bool,

        /// Whether this intrinsic import is async-lowered.
        ///
        /// This is only valid for read/write/cancel intrinsics.
        async_: bool,
    },

    /// A stream-related intrinsic is being imported.
    StreamIntrinsic {
        /// The optional interface to import from, same as `WasmImport::Func`.
        interface: Option<&'a WorldKey>,

        /// The function whose signature this stream type appears in.
        func: &'a Function,

        /// The stream type appearing in `func.find_futures_and_streams(resolve)`.
        ///
        /// Use `None` for the special `unit` payload case.
        ty: Option<TypeId>,

        /// The intrinsic that's being imported.
        intrinsic: StreamIntrinsic,

        /// Whether this import is for an exported WIT function.
        ///
        /// This controls whether the module name is prefixed with `[export]`.
        exported: bool,

        /// Whether this intrinsic import is async-lowered.
        ///
        /// This is only valid for read/write/cancel intrinsics.
        async_: bool,
    },
}

/// Intrinsic definitions to go with [`WasmImport::ResourceIntrinsic`] which
/// also goes with [`Resolve::wasm_import_name`].
#[derive(Debug)]
pub enum ResourceIntrinsic {
    ImportedDrop,
    ExportedDrop,
    ExportedNew,
    ExportedRep,
}

/// Intrinsic definitions to go with [`WasmImport::FutureIntrinsic`] which
/// also goes with [`Resolve::wasm_import_name`].
#[derive(Debug)]
pub enum FutureIntrinsic {
    New,
    Read,
    Write,
    CancelRead,
    CancelWrite,
    DropReadable,
    DropWritable,
}

/// Intrinsic definitions to go with [`WasmImport::StreamIntrinsic`] which
/// also goes with [`Resolve::wasm_import_name`].
#[derive(Debug)]
pub enum StreamIntrinsic {
    New,
    Read,
    Write,
    CancelRead,
    CancelWrite,
    DropReadable,
    DropWritable,
}

/// Indicates whether a function export is a normal export, a post-return
/// function, or a callback function.
#[derive(Debug)]
pub enum WasmExportKind {
    /// Normal function export.
    Normal,

    /// Post-return function.
    PostReturn,

    /// Async callback function.
    Callback,
}

/// Different kinds of exports that can be passed to
/// [`Resolve::wasm_export_name`] to export from core wasm modules.
#[derive(Debug)]
pub enum WasmExport<'a> {
    /// A WIT function is being exported, optionally from an interface.
    Func {
        /// An optional interface which owns `func`. Use `None` for top-level
        /// world function.
        interface: Option<&'a WorldKey>,

        /// The function being exported.
        func: &'a Function,

        /// Kind of function (normal, post-return, or callback) being exported.
        kind: WasmExportKind,
    },

    /// A destructor for a resource exported from this module.
    ResourceDtor {
        /// The interface that owns the resource.
        interface: &'a WorldKey,
        /// The resource itself that the destructor is for.
        resource: TypeId,
    },

    /// Linear memory, the one that the canonical ABI uses.
    Memory,

    /// An initialization function (not the core wasm `start`).
    Initialize,

    /// The general-purpose realloc hook.
    Realloc,
}

/// Structure returned by [`Resolve::merge`] which contains mappings from
/// old-ids to new-ids after the merge.
#[derive(Default)]
pub struct Remap {
    pub types: Vec<Option<TypeId>>,
    pub interfaces: Vec<Option<InterfaceId>>,
    pub worlds: Vec<Option<WorldId>>,
    pub packages: Vec<PackageId>,

    /// A cache of anonymous `own<T>` handles for resource types.
    ///
    /// The appending operation of `Remap` is the one responsible for
    /// translating references to `T` where `T` is a resource into `own<T>`
    /// instead. This map is used to deduplicate the `own<T>` types generated
    /// to generate as few as possible.
    ///
    /// The key of this map is the resource id `T` in the new resolve, and
    /// the value is the `own<T>` type pointing to `T`.
    own_handles: HashMap<TypeId, TypeId>,

    type_has_borrow: Vec<Option<bool>>,
}

fn apply_map<T>(map: &[Option<Id<T>>], id: Id<T>, desc: &str, span: Span) -> ResolveResult<Id<T>> {
    match map.get(id.index()) {
        Some(Some(id)) => Ok(*id),
        Some(None) => {
            let msg = format!(
                "found a reference to a {desc} which is excluded \
                 due to its feature not being activated"
            );
            Err(ResolveError::new_semantic(span, msg))
        }
        None => panic!("request to remap a {desc} that has not yet been registered"),
    }
}

fn rename(original_name: &str, include_name: &IncludeName) -> Option<String> {
    if original_name == include_name.name {
        return Some(include_name.as_.to_string());
    }
    let (kind, rest) = original_name.split_once(']')?;
    match rest.split_once('.') {
        Some((name, rest)) if name == include_name.name => {
            Some(format!("{kind}]{}.{rest}", include_name.as_))
        }
        _ if rest == include_name.name => Some(format!("{kind}]{}", include_name.as_)),
        _ => None,
    }
}

impl Remap {
    pub fn map_type(&self, id: TypeId, span: Span) -> ResolveResult<TypeId> {
        apply_map(&self.types, id, "type", span)
    }

    pub fn map_interface(&self, id: InterfaceId, span: Span) -> ResolveResult<InterfaceId> {
        apply_map(&self.interfaces, id, "interface", span)
    }

    pub fn map_world(&self, id: WorldId, span: Span) -> ResolveResult<WorldId> {
        apply_map(&self.worlds, id, "world", span)
    }

    pub fn map_world_for_type(&self, id: WorldId, span: Span) -> ResolveResult<WorldId> {
        self.map_world(id, span).map_err(|e| {
            ResolveError::new_semantic(
                span,
                format!("{e}; this type is not gated by a feature but its world is"),
            )
        })
    }

    pub fn map_interface_for_type(
        &self,
        id: InterfaceId,
        span: Span,
    ) -> ResolveResult<InterfaceId> {
        self.map_interface(id, span).map_err(|e| {
            ResolveError::new_semantic(
                span,
                format!("{e}; this type is not gated by a feature but its interface is"),
            )
        })
    }

    fn append(
        &mut self,
        resolve: &mut Resolve,
        unresolved: UnresolvedPackage,
    ) -> ResolveResult<PackageId> {
        let pkgid = resolve.packages.alloc(Package {
            name: unresolved.name.clone(),
            docs: unresolved.docs.clone(),
            interfaces: Default::default(),
            worlds: Default::default(),
        });
        assert!(
            !resolve.package_names.contains_key(&unresolved.name),
            "attempting to re-add package `{}` when it's already present in this `Resolve`",
            unresolved.name,
        );
        resolve.package_names.insert(unresolved.name.clone(), pkgid);
        self.process_foreign_deps(resolve, pkgid, &unresolved)?;

        let foreign_types = self.types.len();
        let foreign_interfaces = self.interfaces.len();
        let foreign_worlds = self.worlds.len();

        // Copy over all types first, updating any intra-type references. Note
        // that types are sorted topologically which means this iteration
        // order should be sufficient. Also note though that the interface
        // owner of a type isn't updated here due to interfaces not being known
        // yet.
        for (id, mut ty) in unresolved.types.into_iter().skip(foreign_types) {
            let span = ty.span;
            if !resolve.include_stability(&ty.stability, &pkgid, span)? {
                self.types.push(None);
                continue;
            }

            self.update_typedef(resolve, &mut ty, span)?;
            let new_id = resolve.types.alloc(ty);
            assert_eq!(self.types.len(), id.index());

            let new_id = match resolve.types[new_id] {
                // If this is an `own<T>` handle then either replace it with a
                // preexisting `own<T>` handle which may have been generated in
                // `update_ty`. If that doesn't exist though then insert it into
                // the `own_handles` cache.
                TypeDef {
                    name: None,
                    owner: TypeOwner::None,
                    kind: TypeDefKind::Handle(Handle::Own(id)),
                    docs: _,
                    stability: _,
                    span: _,
                    external_id: _,
                } => *self.own_handles.entry(id).or_insert(new_id),

                // Everything not-related to `own<T>` doesn't get its ID
                // modified.
                _ => new_id,
            };
            self.types.push(Some(new_id));
        }

        // Next transfer all interfaces into `Resolve`, updating type ids
        // referenced along the way.
        for (id, mut iface) in unresolved.interfaces.into_iter().skip(foreign_interfaces) {
            let span = iface.span;
            if !resolve.include_stability(&iface.stability, &pkgid, span)? {
                self.interfaces.push(None);
                continue;
            }
            assert!(iface.package.is_none());
            iface.package = Some(pkgid);
            self.update_interface(resolve, &mut iface)?;
            let new_id = resolve.interfaces.alloc(iface);
            assert_eq!(self.interfaces.len(), id.index());
            self.interfaces.push(Some(new_id));
        }

        // Now that interfaces are identified go back through the types and
        // update their interface owners.
        for id in self.types.iter().skip(foreign_types) {
            let id = match id {
                Some(id) => *id,
                None => continue,
            };
            let span = resolve.types[id].span;
            match &mut resolve.types[id].owner {
                TypeOwner::Interface(iface_id) => {
                    *iface_id = self.map_interface_for_type(*iface_id, span)?;
                }
                TypeOwner::World(_) | TypeOwner::None => {}
            }
        }

        // Expand worlds. Note that this does NOT process `include` statements,
        // that's handled below. Instead this just handles world item updates
        // and resolves references to types/items within `Resolve`.
        //
        // This is done after types/interfaces are fully settled so the
        // transitive relation between interfaces, through types, is understood
        // here.
        for (id, mut world) in unresolved.worlds.into_iter().skip(foreign_worlds) {
            let world_span = world.span;
            if !resolve.include_stability(&world.stability, &pkgid, world_span)? {
                self.worlds.push(None);
                continue;
            }
            self.update_world(&mut world, resolve, &pkgid)?;

            let new_id = resolve.worlds.alloc(world);
            assert_eq!(self.worlds.len(), id.index());
            self.worlds.push(Some(new_id));
        }

        // As with interfaces, now update the ids of world-owned types.
        for id in self.types.iter().skip(foreign_types) {
            let id = match id {
                Some(id) => *id,
                None => continue,
            };
            let span = resolve.types[id].span;
            match &mut resolve.types[id].owner {
                TypeOwner::World(world_id) => {
                    *world_id = self.map_world_for_type(*world_id, span)?;
                }
                TypeOwner::Interface(_) | TypeOwner::None => {}
            }
        }

        // After the above, process `include` statements for worlds and
        // additionally fully elaborate them. Processing of `include` is
        // deferred until after the steps above so the fully resolved state of
        // local types in this package are all available. This is required
        // because `include` may copy types between worlds when the type is
        // defined in the world itself.
        //
        // This step, after processing `include`, will also use
        // `elaborate_world` to fully expand the world in terms of
        // imports/exports and ensure that all necessary imports/exports are all
        // listed.
        //
        // Note that `self.worlds` is already sorted in topological order so if
        // one world refers to another via `include` then it's guaranteed that
        // the one we're referring to is already expanded and ready to be
        // included.
        for id in self.worlds.iter().skip(foreign_worlds) {
            let Some(id) = *id else {
                continue;
            };
            self.process_world_includes(id, resolve, &pkgid)?;

            let world_span = resolve.worlds[id].span;
            resolve.elaborate_world(id, world_span)?;
        }

        // Fixup "parent" ids now that everything has been identified
        for id in self.interfaces.iter().skip(foreign_interfaces) {
            let id = match id {
                Some(id) => *id,
                None => continue,
            };
            let iface = &mut resolve.interfaces[id];
            iface.package = Some(pkgid);
            if let Some(name) = &iface.name {
                let prev = resolve.packages[pkgid].interfaces.insert(name.clone(), id);
                assert!(prev.is_none());
            }
        }
        for id in self.worlds.iter().skip(foreign_worlds) {
            let id = match id {
                Some(id) => *id,
                None => continue,
            };
            let world = &mut resolve.worlds[id];
            world.package = Some(pkgid);
            let prev = resolve.packages[pkgid]
                .worlds
                .insert(world.name.clone(), id);
            assert!(prev.is_none());
        }
        Ok(pkgid)
    }

    fn process_foreign_deps(
        &mut self,
        resolve: &mut Resolve,
        pkgid: PackageId,
        unresolved: &UnresolvedPackage,
    ) -> ResolveResult<()> {
        // Invert the `foreign_deps` map to be keyed by world id to get
        // used in the loops below.
        let mut world_to_package = HashMap::new();
        let mut interface_to_package = HashMap::new();
        for (i, (pkg_name, worlds_or_ifaces)) in unresolved.foreign_deps.iter().enumerate() {
            for (name, (item, stabilities)) in worlds_or_ifaces {
                match item {
                    AstItem::Interface(unresolved_interface_id) => {
                        let prev = interface_to_package.insert(
                            *unresolved_interface_id,
                            (pkg_name, name, unresolved.foreign_dep_spans[i], stabilities),
                        );
                        assert!(prev.is_none());
                    }
                    AstItem::World(unresolved_world_id) => {
                        let prev = world_to_package.insert(
                            *unresolved_world_id,
                            (pkg_name, name, unresolved.foreign_dep_spans[i], stabilities),
                        );
                        assert!(prev.is_none());
                    }
                }
            }
        }

        // Connect all interfaces referred to in `interface_to_package`, which
        // are at the front of `unresolved.interfaces`, to interfaces already
        // contained within `resolve`.
        self.process_foreign_interfaces(unresolved, &interface_to_package, resolve, &pkgid)?;

        // Connect all worlds referred to in `world_to_package`, which
        // are at the front of `unresolved.worlds`, to worlds already
        // contained within `resolve`.
        self.process_foreign_worlds(unresolved, &world_to_package, resolve, &pkgid)?;

        // Finally, iterate over all foreign-defined types and determine
        // what they map to.
        self.process_foreign_types(unresolved, pkgid, resolve)?;

        for (id, span) in unresolved.required_resource_types.iter() {
            // Note that errors are ignored here because an error represents a
            // type that has been configured away. If a type is configured away
            // then any future use of it will generate an error so there's no
            // need to validate that it's a resource here.
            let Ok(mut id) = self.map_type(*id, *span) else {
                continue;
            };
            loop {
                match resolve.types[id].kind {
                    TypeDefKind::Type(Type::Id(i)) => id = i,
                    TypeDefKind::Resource => break,
                    _ => {
                        return Err(ResolveError::new_semantic(
                            *span,
                            "type used in a handle must be a resource",
                        ));
                    }
                }
            }
        }

        #[cfg(debug_assertions)]
        resolve.assert_valid();

        Ok(())
    }

    fn process_foreign_interfaces(
        &mut self,
        unresolved: &UnresolvedPackage,
        interface_to_package: &HashMap<InterfaceId, (&PackageName, &String, Span, &Vec<Stability>)>,
        resolve: &mut Resolve,
        parent_pkg_id: &PackageId,
    ) -> ResolveResult<()> {
        for (unresolved_iface_id, unresolved_iface) in unresolved.interfaces.iter() {
            let (pkg_name, interface, span, stabilities) =
                match interface_to_package.get(&unresolved_iface_id) {
                    Some(items) => *items,
                    // All foreign interfaces are defined first, so the first one
                    // which is defined in a non-foreign document means that all
                    // further interfaces will be non-foreign as well.
                    None => break,
                };

            let pkgid = resolve
                .package_names
                .get(pkg_name)
                .copied()
                .ok_or_else(|| {
                    ResolveError::from(ResolveErrorKind::PackageNotFound {
                        span,
                        requested: pkg_name.clone(),
                        known: resolve.package_names.keys().cloned().collect(),
                    })
                })?;

            // Functions can't be imported so this should be empty.
            assert!(unresolved_iface.functions.is_empty());

            let pkg = &resolve.packages[pkgid];
            let iface_span = unresolved_iface.span;

            let mut enabled = false;
            for stability in stabilities {
                if resolve.include_stability(stability, parent_pkg_id, iface_span)? {
                    enabled = true;
                    break;
                }
            }

            if !enabled {
                self.interfaces.push(None);
                continue;
            }

            let iface_id = pkg.interfaces.get(interface).copied().ok_or_else(|| {
                ResolveError::new_semantic(iface_span, "interface not found in package")
            })?;
            assert_eq!(self.interfaces.len(), unresolved_iface_id.index());
            self.interfaces.push(Some(iface_id));
        }
        for (id, _) in unresolved.interfaces.iter().skip(self.interfaces.len()) {
            assert!(
                interface_to_package.get(&id).is_none(),
                "found foreign interface after local interface"
            );
        }
        Ok(())
    }

    fn process_foreign_worlds(
        &mut self,
        unresolved: &UnresolvedPackage,
        world_to_package: &HashMap<WorldId, (&PackageName, &String, Span, &Vec<Stability>)>,
        resolve: &mut Resolve,
        parent_pkg_id: &PackageId,
    ) -> ResolveResult<()> {
        for (unresolved_world_id, unresolved_world) in unresolved.worlds.iter() {
            let (pkg_name, world, span, stabilities) =
                match world_to_package.get(&unresolved_world_id) {
                    Some(items) => *items,
                    // Same as above, all worlds are foreign until we find a
                    // non-foreign one.
                    None => break,
                };

            let pkgid = resolve
                .package_names
                .get(pkg_name)
                .copied()
                .ok_or_else(|| {
                    ResolveError::from(ResolveErrorKind::PackageNotFound {
                        span,
                        requested: pkg_name.clone(),
                        known: resolve.package_names.keys().cloned().collect(),
                    })
                })?;
            let pkg = &resolve.packages[pkgid];
            let world_span = unresolved_world.span;

            let mut enabled = false;
            for stability in stabilities {
                if resolve.include_stability(stability, parent_pkg_id, world_span)? {
                    enabled = true;
                    break;
                }
            }

            if !enabled {
                self.worlds.push(None);
                continue;
            }

            let world_id = pkg.worlds.get(world).copied().ok_or_else(|| {
                ResolveError::new_semantic(world_span, "world not found in package")
            })?;
            assert_eq!(self.worlds.len(), unresolved_world_id.index());
            self.worlds.push(Some(world_id));
        }
        for (id, _) in unresolved.worlds.iter().skip(self.worlds.len()) {
            assert!(
                world_to_package.get(&id).is_none(),
                "found foreign world after local world"
            );
        }
        Ok(())
    }

    fn process_foreign_types(
        &mut self,
        unresolved: &UnresolvedPackage,
        pkgid: PackageId,
        resolve: &mut Resolve,
    ) -> ResolveResult<()> {
        for (unresolved_type_id, unresolved_ty) in unresolved.types.iter() {
            // All "Unknown" types should appear first so once we're no longer
            // in unknown territory it's package-defined types so break out of
            // this loop.
            match unresolved_ty.kind {
                TypeDefKind::Unknown => {}
                _ => break,
            }

            let span = unresolved_ty.span;
            if !resolve.include_stability(&unresolved_ty.stability, &pkgid, span)? {
                self.types.push(None);
                continue;
            }

            let unresolved_iface_id = match unresolved_ty.owner {
                TypeOwner::Interface(id) => id,
                _ => unreachable!(),
            };
            let iface_id = self.map_interface(unresolved_iface_id, Default::default())?;
            let name = unresolved_ty.name.as_ref().unwrap();
            let span = unresolved.unknown_type_spans[unresolved_type_id.index()];
            let type_id = *resolve.interfaces[iface_id]
                .types
                .get(name)
                .ok_or_else(|| {
                    ResolveError::new_semantic(
                        span,
                        format!("type `{name}` not defined in interface"),
                    )
                })?;
            assert_eq!(self.types.len(), unresolved_type_id.index());
            self.types.push(Some(type_id));
        }
        for (_, ty) in unresolved.types.iter().skip(self.types.len()) {
            if let TypeDefKind::Unknown = ty.kind {
                panic!("unknown type after defined type");
            }
        }
        Ok(())
    }

    fn update_typedef(
        &mut self,
        resolve: &mut Resolve,
        ty: &mut TypeDef,
        span: Span,
    ) -> ResolveResult<()> {
        // NB: note that `ty.owner` is not updated here since interfaces
        // haven't been mapped yet and that's done in a separate step.
        use crate::TypeDefKind::*;
        match &mut ty.kind {
            Handle(handle) => match handle {
                crate::Handle::Own(ty) | crate::Handle::Borrow(ty) => {
                    self.update_type_id(ty, span)?
                }
            },
            Resource => {}
            Record(r) => {
                for field in r.fields.iter_mut() {
                    self.update_ty(resolve, &mut field.ty, field.span)?
                }
            }
            Tuple(t) => {
                for ty in t.types.iter_mut() {
                    self.update_ty(resolve, ty, span)?;
                }
            }
            Variant(v) => {
                for case in v.cases.iter_mut() {
                    if let Some(t) = &mut case.ty {
                        self.update_ty(resolve, t, span)?;
                    }
                }
            }
            Option(t)
            | List(t, ..)
            | FixedLengthList(t, ..)
            | Future(Some(t))
            | Stream(Some(t)) => self.update_ty(resolve, t, span)?,
            Map(k, v) => {
                self.update_ty(resolve, k, span)?;
                self.update_ty(resolve, v, span)?;
            }
            Result(r) => {
                if let Some(ty) = &mut r.ok {
                    self.update_ty(resolve, ty, span)?;
                }
                if let Some(ty) = &mut r.err {
                    self.update_ty(resolve, ty, span)?;
                }
            }

            // Note that `update_ty` is specifically not used here as typedefs
            // because for the `type a = b` form that doesn't force `a` to be a
            // handle type if `b` is a resource type, instead `a` is
            // simultaneously usable as a resource and a handle type
            Type(crate::Type::Id(id)) => self.update_type_id(id, span)?,
            Type(_) => {}

            // nothing to do for these as they're just names or empty
            Flags(_) | Enum(_) | Future(None) | Stream(None) => {}

            Unknown => unreachable!(),
        }

        Ok(())
    }

    fn update_ty(&mut self, resolve: &mut Resolve, ty: &mut Type, span: Span) -> ResolveResult<()> {
        let id = match ty {
            Type::Id(id) => id,
            _ => return Ok(()),
        };
        self.update_type_id(id, span)?;

        // If `id` points to a `Resource` type then this means that what was
        // just discovered was a reference to what will implicitly become an
        // `own<T>` handle. This `own` handle is implicitly allocated here
        // and handled during the merging process.
        let mut cur = *id;
        let points_to_resource = loop {
            match resolve.types[cur].kind {
                TypeDefKind::Type(Type::Id(id)) => cur = id,
                TypeDefKind::Resource => break true,
                _ => break false,
            }
        };

        if points_to_resource {
            *id = *self.own_handles.entry(*id).or_insert_with(|| {
                resolve.types.alloc(TypeDef {
                    name: None,
                    owner: TypeOwner::None,
                    kind: TypeDefKind::Handle(Handle::Own(*id)),
                    docs: Default::default(),
                    stability: Default::default(),
                    span: Default::default(),
                    external_id: None,
                })
            });
        }
        Ok(())
    }

    fn update_type_id(&self, id: &mut TypeId, span: Span) -> ResolveResult<()> {
        *id = self.map_type(*id, span)?;
        Ok(())
    }

    fn update_interface(
        &mut self,
        resolve: &mut Resolve,
        iface: &mut Interface,
    ) -> ResolveResult<()> {
        iface.types.retain(|_, ty| self.types[ty.index()].is_some());
        let iface_pkg_id = iface.package.as_ref().unwrap_or_else(|| {
            panic!(
                "unexpectedly missing package on interface [{}]",
                iface
                    .name
                    .as_ref()
                    .map(String::as_str)
                    .unwrap_or("<unknown>"),
            )
        });

        // NB: note that `iface.doc` is not updated here since interfaces
        // haven't been mapped yet and that's done in a separate step.
        for (_name, ty) in iface.types.iter_mut() {
            self.update_type_id(ty, iface.span)?;
        }
        for (_, func) in iface.functions.iter_mut() {
            let span = func.span;
            if !resolve.include_stability(&func.stability, iface_pkg_id, span)? {
                continue;
            }
            self.update_function(resolve, func, span)?
        }

        // Filter out all of the existing functions in interface which fail the
        // `include_stability()` check, as they shouldn't be available.
        for (name, func) in mem::take(&mut iface.functions) {
            if resolve.include_stability(&func.stability, iface_pkg_id, func.span)? {
                iface.functions.insert(name, func);
            }
        }

        Ok(())
    }

    fn update_function(
        &mut self,
        resolve: &mut Resolve,
        func: &mut Function,
        span: Span,
    ) -> ResolveResult<()> {
        if let Some(id) = func.kind.resource_mut() {
            self.update_type_id(id, span)?;
        }
        for param in func.params.iter_mut() {
            self.update_ty(resolve, &mut param.ty, span)?;
        }
        if let Some(ty) = &mut func.result {
            self.update_ty(resolve, ty, span)?;
        }

        if let Some(ty) = &func.result {
            if self.type_has_borrow(resolve, ty) {
                return Err(ResolveError::new_semantic(
                    span,
                    format!(
                        "function `{}` returns a type which contains a `borrow<T>` which is not supported",
                        func.name,
                    ),
                ));
            }
        }

        Ok(())
    }

    fn update_world(
        &mut self,
        world: &mut World,
        resolve: &mut Resolve,
        pkg_id: &PackageId,
    ) -> ResolveResult<()> {
        // Rewrite imports/exports with their updated versions. Note that this
        // may involve updating the key of the imports/exports maps so this
        // starts by emptying them out and then everything is re-inserted.
        let imports = mem::take(&mut world.imports).into_iter().map(|p| (p, true));
        let exports = mem::take(&mut world.exports)
            .into_iter()
            .map(|p| (p, false));
        for ((mut name, mut item), import) in imports.chain(exports) {
            let span = item.span();
            // Update the `id` eagerly here so `item.stability(..)` below
            // works.
            if let WorldItem::Type { id, .. } = &mut item {
                *id = self.map_type(*id, span)?;
            }
            let stability = item.stability(resolve);
            if !resolve.include_stability(stability, pkg_id, span)? {
                continue;
            }
            self.update_world_key(&mut name, span)?;
            match &mut item {
                WorldItem::Interface { id, .. } => {
                    *id = self.map_interface(*id, span)?;
                }
                WorldItem::Function(f) => {
                    self.update_function(resolve, f, span)?;
                }
                WorldItem::Type { .. } => {
                    // already mapped above
                }
            }

            let dst = if import {
                &mut world.imports
            } else {
                &mut world.exports
            };
            let prev = dst.insert(name, item);
            assert!(prev.is_none());
        }

        Ok(())
    }

    fn process_world_includes(
        &self,
        id: WorldId,
        resolve: &mut Resolve,
        pkg_id: &PackageId,
    ) -> ResolveResult<()> {
        let world = &mut resolve.worlds[id];
        // Resolve all `include` statements of the world which will add more
        // entries to the imports/exports list for this world.
        let includes = mem::take(&mut world.includes);
        for include in includes {
            if !resolve.include_stability(&include.stability, pkg_id, include.span)? {
                continue;
            }
            self.resolve_include(
                id,
                include.id,
                &include.names,
                include.span,
                pkg_id,
                resolve,
            )?;
        }

        // Validate that there are no case-insensitive duplicate names in imports/exports
        Self::validate_world_case_insensitive_names(resolve, id)?;

        Ok(())
    }

    /// Validates that a world's imports and exports don't have case-insensitive
    /// duplicate names. Per the WIT specification, kebab-case identifiers are
    /// case-insensitive within the same scope.
    fn validate_world_case_insensitive_names(
        resolve: &Resolve,
        world_id: WorldId,
    ) -> ResolveResult<()> {
        let world = &resolve.worlds[world_id];

        // Helper closure to check for case-insensitive duplicates in a map
        let validate_names =
            |items: &IndexMap<WorldKey, WorldItem>, item_type: &str| -> ResolveResult<()> {
                let mut seen_lowercase: HashMap<String, String> = HashMap::new();

                for key in items.keys() {
                    // Only WorldKey::Name variants can have case-insensitive conflicts
                    if let WorldKey::Name(name) = key {
                        let lowercase_name = name.to_lowercase();

                        if let Some(existing_name) = seen_lowercase.get(&lowercase_name) {
                            // Only error on case-insensitive duplicates (e.g., "foo" vs "FOO").
                            // Exact duplicates would have been caught earlier.
                            if existing_name != name {
                                // TODO: `WorldKey::Name` does not carry a `Span`, so we
                                // cannot point at the conflicting item. Add a span to
                                // `WorldKey::Name` to improve this error.
                                return Err(ResolveError::new_semantic(
                                    Span::default(),
                                    format!(
                                        "{item_type} `{name}` in world `{}` conflicts with \
                                     {item_type} `{existing_name}` \
                                     (kebab-case identifiers are case-insensitive)",
                                        world.name,
                                    ),
                                ));
                            }
                        }

                        seen_lowercase.insert(lowercase_name, name.clone());
                    }
                }

                Ok(())
            };

        validate_names(&world.imports, "import")?;
        validate_names(&world.exports, "export")?;

        Ok(())
    }

    fn update_world_key(&self, key: &mut WorldKey, span: Span) -> ResolveResult<()> {
        match key {
            WorldKey::Name(_) => {}
            WorldKey::Interface(id) => {
                *id = self.map_interface(*id, span)?;
            }
        }
        Ok(())
    }

    fn resolve_include(
        &self,
        id: WorldId,
        include_world_id_orig: WorldId,
        names: &[IncludeName],
        span: Span,
        pkg_id: &PackageId,
        resolve: &mut Resolve,
    ) -> ResolveResult<()> {
        let world = &resolve.worlds[id];
        let include_world_id = self.map_world(include_world_id_orig, span)?;
        let include_world = resolve.worlds[include_world_id].clone();
        let mut names_ = names.to_owned();
        let is_external_include = world.package != include_world.package;

        // remove all imports and exports that match the names we're including
        for import in include_world.imports.iter() {
            self.remove_matching_name(import, &mut names_);
        }
        for export in include_world.exports.iter() {
            self.remove_matching_name(export, &mut names_);
        }
        if !names_.is_empty() {
            return Err(ResolveError::new_semantic(
                span,
                format!(
                    "no import or export kebab-name `{}`. Note that an ID does not support renaming",
                    names_[0].name
                ),
            ));
        }

        let mut maps = Default::default();
        let mut cloner = clone::Cloner::new(
            resolve,
            &mut maps,
            TypeOwner::World(if is_external_include {
                include_world_id
            } else {
                include_world_id
                // include_world_id_orig
            }),
            TypeOwner::World(id),
        );
        cloner.new_package = Some(*pkg_id);

        // copy the imports and exports from the included world into the current world
        for import in include_world.imports.iter() {
            self.resolve_include_item(
                &mut cloner,
                names,
                |resolve| &mut resolve.worlds[id].imports,
                import,
                span,
                "import",
                is_external_include,
            )?;
        }

        for export in include_world.exports.iter() {
            self.resolve_include_item(
                &mut cloner,
                names,
                |resolve| &mut resolve.worlds[id].exports,
                export,
                span,
                "export",
                is_external_include,
            )?;
        }
        Ok(())
    }

    fn resolve_include_item(
        &self,
        cloner: &mut clone::Cloner<'_>,
        names: &[IncludeName],
        get_items: impl Fn(&mut Resolve) -> &mut IndexMap<WorldKey, WorldItem>,
        item: (&WorldKey, &WorldItem),
        span: Span,
        item_type: &str,
        is_external_include: bool,
    ) -> ResolveResult<()> {
        match item.0 {
            WorldKey::Name(n) => {
                let n = names
                    .into_iter()
                    .find_map(|include_name| rename(n, include_name))
                    .unwrap_or(n.clone());

                // When the `with` option to the `include` directive is
                // specified and is used to rename a function that means that
                // the function's own original name needs to be updated, so
                // reflect the change not only in the world key but additionally
                // in the function itself.
                let mut new_item = item.1.clone();
                let key = WorldKey::Name(n.clone());
                cloner.world_item(&key, &mut new_item);
                match &mut new_item {
                    WorldItem::Function(f) => f.name = n.clone(),
                    WorldItem::Type { id, .. } => cloner.resolve.types[*id].name = Some(n.clone()),
                    WorldItem::Interface { .. } => {}
                }

                let prev = get_items(cloner.resolve).insert(key, new_item);
                if prev.is_some() {
                    return Err(ResolveError::from(ResolveErrorKind::ItemShadowing {
                        span,
                        item_type: item_type.to_owned(),
                        name: n,
                    }));
                }
            }
            key @ WorldKey::Interface(_) => {
                let prev = get_items(cloner.resolve)
                    .entry(key.clone())
                    .or_insert(item.1.clone());
                match (&item.1, prev) {
                    (
                        WorldItem::Interface {
                            id: aid,
                            stability: astability,
                            span: aspan,
                            ..
                        },
                        WorldItem::Interface {
                            id: bid,
                            stability: bstability,
                            ..
                        },
                    ) => {
                        assert_eq!(*aid, *bid);
                        merge_include_stability(
                            astability,
                            bstability,
                            is_external_include,
                            *aspan,
                        )?;
                    }
                    (WorldItem::Interface { .. }, _) => unreachable!(),
                    (WorldItem::Function(_), _) => unreachable!(),
                    (WorldItem::Type { .. }, _) => unreachable!(),
                }
            }
        };

        Ok(())
    }

    fn remove_matching_name(&self, item: (&WorldKey, &WorldItem), names: &mut Vec<IncludeName>) {
        match item.0 {
            WorldKey::Name(n) => {
                names.retain(|name| rename(n, name).is_none());
            }
            _ => {}
        }
    }

    fn type_has_borrow(&mut self, resolve: &Resolve, ty: &Type) -> bool {
        let id = match ty {
            Type::Id(id) => *id,
            _ => return false,
        };

        if let Some(Some(has_borrow)) = self.type_has_borrow.get(id.index()) {
            return *has_borrow;
        }

        let result = self.typedef_has_borrow(resolve, &resolve.types[id]);
        if self.type_has_borrow.len() <= id.index() {
            self.type_has_borrow.resize(id.index() + 1, None);
        }
        self.type_has_borrow[id.index()] = Some(result);
        result
    }

    fn typedef_has_borrow(&mut self, resolve: &Resolve, ty: &TypeDef) -> bool {
        match &ty.kind {
            TypeDefKind::Type(t) => self.type_has_borrow(resolve, t),
            TypeDefKind::Variant(v) => v
                .cases
                .iter()
                .filter_map(|case| case.ty.as_ref())
                .any(|ty| self.type_has_borrow(resolve, ty)),
            TypeDefKind::Handle(Handle::Borrow(_)) => true,
            TypeDefKind::Handle(Handle::Own(_)) => false,
            TypeDefKind::Resource => false,
            TypeDefKind::Record(r) => r
                .fields
                .iter()
                .any(|case| self.type_has_borrow(resolve, &case.ty)),
            TypeDefKind::Flags(_) => false,
            TypeDefKind::Tuple(t) => t.types.iter().any(|t| self.type_has_borrow(resolve, t)),
            TypeDefKind::Enum(_) => false,
            TypeDefKind::List(ty)
            | TypeDefKind::FixedLengthList(ty, ..)
            | TypeDefKind::Future(Some(ty))
            | TypeDefKind::Stream(Some(ty))
            | TypeDefKind::Option(ty) => self.type_has_borrow(resolve, ty),
            TypeDefKind::Map(k, v) => {
                self.type_has_borrow(resolve, k) || self.type_has_borrow(resolve, v)
            }
            TypeDefKind::Result(r) => [&r.ok, &r.err]
                .iter()
                .filter_map(|t| t.as_ref())
                .any(|t| self.type_has_borrow(resolve, t)),
            TypeDefKind::Future(None) | TypeDefKind::Stream(None) => false,
            TypeDefKind::Unknown => unreachable!(),
        }
    }
}

struct MergeMap<'a> {
    /// A map of package ids in `from` to those in `into` for those that are
    /// found to be equivalent.
    package_map: HashMap<PackageId, PackageId>,

    /// A map of interface ids in `from` to those in `into` for those that are
    /// found to be equivalent.
    interface_map: HashMap<InterfaceId, InterfaceId>,

    /// A map of type ids in `from` to those in `into` for those that are
    /// found to be equivalent.
    type_map: HashMap<TypeId, TypeId>,

    /// A map of world ids in `from` to those in `into` for those that are
    /// found to be equivalent.
    world_map: HashMap<WorldId, WorldId>,

    /// A list of documents that need to be added to packages in `into`.
    ///
    /// The elements here are:
    ///
    /// * The name of the interface/world
    /// * The ID within `into` of the package being added to
    /// * The ID within `from` of the item being added.
    interfaces_to_add: Vec<(String, PackageId, InterfaceId)>,
    worlds_to_add: Vec<(String, PackageId, WorldId)>,

    /// Which `Resolve` is being merged from.
    from: &'a Resolve,

    /// Which `Resolve` is being merged into.
    into: &'a Resolve,
}

impl<'a> MergeMap<'a> {
    fn new(from: &'a Resolve, into: &'a Resolve) -> MergeMap<'a> {
        MergeMap {
            package_map: Default::default(),
            interface_map: Default::default(),
            type_map: Default::default(),
            world_map: Default::default(),
            interfaces_to_add: Default::default(),
            worlds_to_add: Default::default(),
            from,
            into,
        }
    }

    fn build(&mut self) -> anyhow::Result<()> {
        for from_id in self.from.topological_packages() {
            let from = &self.from.packages[from_id];
            let into_id = match self.into.package_names.get(&from.name) {
                Some(id) => *id,

                // This package, according to its name and url, is not present
                // in `self` so it needs to get added below.
                None => {
                    log::trace!("adding unique package {}", from.name);
                    continue;
                }
            };
            log::trace!("merging duplicate package {}", from.name);

            self.build_package(from_id, into_id).with_context(|| {
                format!("failed to merge package `{}` into existing copy", from.name)
            })?;
        }

        Ok(())
    }

    fn build_package(&mut self, from_id: PackageId, into_id: PackageId) -> anyhow::Result<()> {
        let prev = self.package_map.insert(from_id, into_id);
        assert!(prev.is_none());

        let from = &self.from.packages[from_id];
        let into = &self.into.packages[into_id];

        // If an interface is present in `from_id` but not present in `into_id`
        // then it can be copied over wholesale. That copy is scheduled to
        // happen within the `self.interfaces_to_add` list.
        for (name, from_interface_id) in from.interfaces.iter() {
            let into_interface_id = match into.interfaces.get(name) {
                Some(id) => *id,
                None => {
                    log::trace!("adding unique interface {name}");
                    self.interfaces_to_add
                        .push((name.clone(), into_id, *from_interface_id));
                    continue;
                }
            };

            log::trace!("merging duplicate interfaces {name}");
            self.build_interface(*from_interface_id, into_interface_id)
                .with_context(|| format!("failed to merge interface `{name}`"))?;
        }

        for (name, from_world_id) in from.worlds.iter() {
            let into_world_id = match into.worlds.get(name) {
                Some(id) => *id,
                None => {
                    log::trace!("adding unique world {name}");
                    self.worlds_to_add
                        .push((name.clone(), into_id, *from_world_id));
                    continue;
                }
            };

            log::trace!("merging duplicate worlds {name}");
            self.build_world(*from_world_id, into_world_id)
                .with_context(|| format!("failed to merge world `{name}`"))?;
        }

        Ok(())
    }

    fn build_interface(
        &mut self,
        from_id: InterfaceId,
        into_id: InterfaceId,
    ) -> anyhow::Result<()> {
        let prev = self.interface_map.insert(from_id, into_id);
        assert!(prev.is_none());

        let from_interface = &self.from.interfaces[from_id];
        let into_interface = &self.into.interfaces[into_id];

        // When merging interfaces, types and functions that exist in both
        // `from` and `into` must match structurally. Either side is allowed
        // to have extra types or functions not present in the other, which
        // enables commutative merging of partial views of the same
        // interface. The only requirement is that the intersection of the
        // two interfaces is compatible.

        for (name, from_type_id) in from_interface.types.iter() {
            let into_type_id = match into_interface.types.get(name) {
                Some(id) => *id,
                // Extra type in `from` not present in `into`; it will be
                // moved as a new type and added to the interface later.
                None => continue,
            };
            let prev = self.type_map.insert(*from_type_id, into_type_id);
            assert!(prev.is_none());

            self.build_type_id(*from_type_id, into_type_id)
                .with_context(|| format!("mismatch in type `{name}`"))?;
        }

        for (name, from_func) in from_interface.functions.iter() {
            let into_func = match into_interface.functions.get(name) {
                Some(func) => func,
                // Extra function in `from` not present in `into`; it will
                // be added to the interface during the merge phase.
                None => continue,
            };
            self.build_function(from_func, into_func)
                .with_context(|| format!("mismatch in function `{name}`"))?;
        }

        Ok(())
    }

    fn build_type_id(&mut self, from_id: TypeId, into_id: TypeId) -> anyhow::Result<()> {
        // FIXME: ideally the types should be "structurally
        // equal" but that's not trivial to do in the face of
        // resources.
        let _ = from_id;
        let _ = into_id;
        Ok(())
    }

    fn build_type(&mut self, from_ty: &Type, into_ty: &Type) -> anyhow::Result<()> {
        match (from_ty, into_ty) {
            (Type::Id(from), Type::Id(into)) => {
                self.build_type_id(*from, *into)?;
            }
            (from, into) if from != into => bail!("different kinds of types"),
            _ => {}
        }
        Ok(())
    }

    fn build_function(&mut self, from_func: &Function, into_func: &Function) -> anyhow::Result<()> {
        if from_func.name != into_func.name {
            bail!(
                "different function names `{}` and `{}`",
                from_func.name,
                into_func.name
            );
        }
        match (&from_func.kind, &into_func.kind) {
            (FunctionKind::Freestanding, FunctionKind::Freestanding) => {}
            (FunctionKind::AsyncFreestanding, FunctionKind::AsyncFreestanding) => {}

            (FunctionKind::Method(from), FunctionKind::Method(into))
            | (FunctionKind::Static(from), FunctionKind::Static(into))
            | (FunctionKind::AsyncMethod(from), FunctionKind::AsyncMethod(into))
            | (FunctionKind::AsyncStatic(from), FunctionKind::AsyncStatic(into))
            | (FunctionKind::Constructor(from), FunctionKind::Constructor(into)) => {
                self.build_type_id(*from, *into)
                    .context("different function kind types")?;
            }

            (FunctionKind::Method(_), _)
            | (FunctionKind::Constructor(_), _)
            | (FunctionKind::Static(_), _)
            | (FunctionKind::Freestanding, _)
            | (FunctionKind::AsyncFreestanding, _)
            | (FunctionKind::AsyncMethod(_), _)
            | (FunctionKind::AsyncStatic(_), _) => {
                bail!("different function kind types")
            }
        }

        if from_func.params.len() != into_func.params.len() {
            bail!("different number of function parameters");
        }
        for (from_param, into_param) in from_func.params.iter().zip(&into_func.params) {
            if from_param.name != into_param.name {
                bail!(
                    "different function parameter names: {} != {}",
                    from_param.name,
                    into_param.name
                );
            }
            self.build_type(&from_param.ty, &into_param.ty)
                .with_context(|| {
                    format!(
                        "different function parameter types for `{}`",
                        from_param.name
                    )
                })?;
        }
        match (&from_func.result, &into_func.result) {
            (Some(from_ty), Some(into_ty)) => {
                self.build_type(from_ty, into_ty)
                    .context("different function result types")?;
            }
            (None, None) => {}
            (Some(_), None) | (None, Some(_)) => bail!("different number of function results"),
        }
        Ok(())
    }

    fn build_world(&mut self, from_id: WorldId, into_id: WorldId) -> anyhow::Result<()> {
        let prev = self.world_map.insert(from_id, into_id);
        assert!(prev.is_none());

        let from_world = &self.from.worlds[from_id];
        let into_world = &self.into.worlds[into_id];

        // Same as interfaces worlds are expected to exactly match to avoid
        // unexpectedly changing a particular component's view of imports and
        // exports.
        //
        // FIXME: this should probably share functionality with
        // `Resolve::merge_worlds` to support adding imports but not changing
        // exports.

        if from_world.imports.len() != into_world.imports.len() {
            bail!("world contains different number of imports than expected");
        }
        if from_world.exports.len() != into_world.exports.len() {
            bail!("world contains different number of exports than expected");
        }

        for (from_name, from) in from_world.imports.iter() {
            let into_name = MergeMap::map_name(from_name, &self.interface_map);
            let name_str = self.from.name_world_key(from_name);
            let into = into_world
                .imports
                .get(&into_name)
                .ok_or_else(|| anyhow!("import `{name_str}` not found in target world"))?;
            self.match_world_item(from, into)
                .with_context(|| format!("import `{name_str}` didn't match target world"))?;
        }

        for (from_name, from) in from_world.exports.iter() {
            let into_name = MergeMap::map_name(from_name, &self.interface_map);
            let name_str = self.from.name_world_key(from_name);
            let into = into_world
                .exports
                .get(&into_name)
                .ok_or_else(|| anyhow!("export `{name_str}` not found in target world"))?;
            self.match_world_item(from, into)
                .with_context(|| format!("export `{name_str}` didn't match target world"))?;
        }

        Ok(())
    }

    fn map_name(
        from_name: &WorldKey,
        interface_map: &HashMap<InterfaceId, InterfaceId>,
    ) -> WorldKey {
        match from_name {
            WorldKey::Name(s) => WorldKey::Name(s.clone()),
            WorldKey::Interface(id) => {
                WorldKey::Interface(interface_map.get(id).copied().unwrap_or(*id))
            }
        }
    }

    fn match_world_item(&mut self, from: &WorldItem, into: &WorldItem) -> anyhow::Result<()> {
        match (from, into) {
            (WorldItem::Interface { id: from, .. }, WorldItem::Interface { id: into, .. }) => {
                match (
                    &self.from.interfaces[*from].name,
                    &self.into.interfaces[*into].name,
                ) {
                    // If one interface is unnamed then they must both be
                    // unnamed and they must both have the same structure for
                    // now.
                    (None, None) => self.build_interface(*from, *into)?,

                    // Otherwise both interfaces must be named and they must
                    // have been previously found to be equivalent. Note that
                    // if either is unnamed it won't be present in
                    // `interface_map` so this'll return an error.
                    _ => {
                        if self.interface_map.get(from) != Some(into) {
                            bail!("interfaces are not the same");
                        }
                    }
                }
            }
            (WorldItem::Function(from), WorldItem::Function(into)) => {
                let _ = (from, into);
                // FIXME: should assert an check that `from` structurally
                // matches `into`
            }
            (WorldItem::Type { id: from, .. }, WorldItem::Type { id: into, .. }) => {
                // FIXME: should assert an check that `from` structurally
                // matches `into`
                let prev = self.type_map.insert(*from, *into);
                assert!(prev.is_none());
            }

            (WorldItem::Interface { .. }, _)
            | (WorldItem::Function(_), _)
            | (WorldItem::Type { .. }, _) => {
                bail!("world items do not have the same type")
            }
        }
        Ok(())
    }
}

/// Updates stability annotations when merging `from` into `into`.
///
/// This is done to keep up-to-date stability information if possible.
/// Components for example don't carry stability information but WIT does so
/// this tries to move from "unknown" to stable/unstable if possible.
fn update_stability(from: &Stability, into: &mut Stability, span: Span) -> ResolveResult<()> {
    // If `from` is unknown or the two stability annotations are equal then
    // there's nothing to do here.
    if from == into || from.is_unknown() {
        return Ok(());
    }
    // Otherwise if `into` is unknown then inherit the stability listed in
    // `from`.
    if into.is_unknown() {
        *into = from.clone();
        return Ok(());
    }

    // Failing all that this means that the two attributes are different so
    // generate an error.
    Err(ResolveError::from(ResolveErrorKind::StabilityMismatch {
        span,
        from: from.clone(),
        into: into.clone(),
    }))
}

fn merge_include_stability(
    from: &Stability,
    into: &mut Stability,
    is_external_include: bool,
    span: Span,
) -> ResolveResult<()> {
    if is_external_include && from.is_stable() {
        log::trace!("dropped stability from external package");
        *into = Stability::Unknown;
        return Ok(());
    }

    update_stability(from, into, span)
}

#[cfg(test)]
mod tests {
    use crate::alloc::format;
    use crate::alloc::string::{String, ToString};
    use crate::alloc::vec::Vec;
    use crate::{Resolve, SourceMap, WorldItem, WorldKey};
    use anyhow::Result;

    #[test]
    fn select_world() -> Result<()> {
        let mut resolve = Resolve::default();
        resolve.push_str(
            "test.wit",
            r#"
                package foo:bar@0.1.0;

                world foo {}
            "#,
        )?;
        resolve.push_str(
            "test.wit",
            r#"
                package foo:baz@0.1.0;

                world foo {}
            "#,
        )?;
        resolve.push_str(
            "test.wit",
            r#"
                package foo:baz@0.2.0;

                world foo {}
            "#,
        )?;

        let dummy = resolve.push_str(
            "test.wit",
            r#"
                package foo:dummy;

                world foo {}
            "#,
        )?;

        assert!(resolve.select_world(&[dummy], None).is_ok());
        assert!(resolve.select_world(&[dummy], Some("xx")).is_err());
        assert!(resolve.select_world(&[dummy], Some("")).is_err());
        assert!(resolve.select_world(&[dummy], Some("foo:bar/foo")).is_ok());
        assert!(
            resolve
                .select_world(&[dummy], Some("foo:bar/foo@0.1.0"))
                .is_ok()
        );
        assert!(resolve.select_world(&[dummy], Some("foo:baz/foo")).is_err());
        assert!(
            resolve
                .select_world(&[dummy], Some("foo:baz/foo@0.1.0"))
                .is_ok()
        );
        assert!(
            resolve
                .select_world(&[dummy], Some("foo:baz/foo@0.2.0"))
                .is_ok()
        );
        Ok(())
    }

    #[test]
    fn wasm_import_name_future_and_stream_intrinsics() -> Result<()> {
        use crate::{FutureIntrinsic, LiftLowerAbi, ManglingAndAbi, StreamIntrinsic, WasmImport};

        let mut resolve = Resolve::default();
        let pkg = resolve.push_str(
            "test.wit",
            r#"
                package foo:bar;

                interface iface {
                    iface-func: func(x: future<u32>) -> stream<u32>;
                }

                world w {
                    import import-func: func(x: future<future<u32>>, y: u32) -> stream<string>;
                    export export-func: func(x: future, y: stream);
                    import iface;
                    export iface;
                }
            "#,
        )?;
        let world = resolve.packages[pkg].worlds["w"];
        let world = &resolve.worlds[world];
        let mangling = ManglingAndAbi::Legacy(LiftLowerAbi::AsyncStackful);

        let WorldItem::Function(import_func) =
            &world.imports[&WorldKey::Name("import-func".to_string())]
        else {
            panic!("expected `import-func` to be a top-level world import");
        };
        let WorldItem::Function(export_func) =
            &world.exports[&WorldKey::Name("export-func".to_string())]
        else {
            panic!("expected `export-func` to be a top-level world export");
        };
        let import_types = import_func.find_futures_and_streams(&resolve);
        assert_eq!(import_types.len(), 3);

        let (interface_key, interface_func) = world
            .imports
            .iter()
            .find_map(|(key, item)| match item {
                WorldItem::Interface { id, .. } => Some((
                    key.clone(),
                    &resolve.interfaces[*id].functions["iface-func"],
                )),
                _ => None,
            })
            .expect("expected interface import");
        let interface_types = interface_func.find_futures_and_streams(&resolve);
        assert_eq!(interface_types.len(), 2);

        let (module, name) = resolve.wasm_import_name(
            mangling,
            WasmImport::FutureIntrinsic {
                interface: None,
                func: import_func,
                ty: Some(import_types[0]),
                intrinsic: FutureIntrinsic::New,
                exported: false,
                async_: false,
            },
        );
        assert_eq!(module, "$root");
        assert_eq!(name, "[future-new-0]import-func");

        let (module, name) = resolve.wasm_import_name(
            mangling,
            WasmImport::FutureIntrinsic {
                interface: None,
                func: import_func,
                ty: Some(import_types[1]),
                intrinsic: FutureIntrinsic::Read,
                exported: false,
                async_: true,
            },
        );
        assert_eq!(module, "$root");
        assert_eq!(name, "[async-lower][future-read-1]import-func");

        let (module, name) = resolve.wasm_import_name(
            mangling,
            WasmImport::StreamIntrinsic {
                interface: None,
                func: import_func,
                ty: Some(import_types[2]),
                intrinsic: StreamIntrinsic::CancelRead,
                exported: false,
                async_: true,
            },
        );
        assert_eq!(module, "$root");
        assert_eq!(name, "[async-lower][stream-cancel-read-2]import-func");

        let (module, name) = resolve.wasm_import_name(
            mangling,
            WasmImport::FutureIntrinsic {
                interface: None,
                func: export_func,
                ty: None,
                intrinsic: FutureIntrinsic::DropReadable,
                exported: true,
                async_: false,
            },
        );
        assert_eq!(module, "[export]$root");
        assert_eq!(name, "[future-drop-readable-unit]export-func");

        let (module, name) = resolve.wasm_import_name(
            mangling,
            WasmImport::StreamIntrinsic {
                interface: None,
                func: export_func,
                ty: None,
                intrinsic: StreamIntrinsic::Write,
                exported: true,
                async_: true,
            },
        );
        assert_eq!(module, "[export]$root");
        assert_eq!(name, "[async-lower][stream-write-unit]export-func");

        let (module, name) = resolve.wasm_import_name(
            mangling,
            WasmImport::StreamIntrinsic {
                interface: Some(&interface_key),
                func: interface_func,
                ty: Some(interface_types[1]),
                intrinsic: StreamIntrinsic::Read,
                exported: true,
                async_: false,
            },
        );
        assert_eq!(
            module,
            format!("[export]{}", resolve.name_world_key(&interface_key))
        );
        assert_eq!(name, "[stream-read-1]iface-func");

        Ok(())
    }

    /// When there are multiple packages and there's no main package, don't
    /// pick a world just based on it being the only one that matches.
    #[test]
    fn select_world_multiple_packages() -> Result<()> {
        use wit_parser::Resolve;

        let mut resolve = Resolve::default();

        // Just one world in one package; we always succeed.
        let stuff = resolve.push_str(
            "./my-test.wit",
            r#"
                    package test:stuff;

                    world foo {
                        // ...
                    }
                "#,
        )?;
        assert!(resolve.select_world(&[stuff], None).is_ok());
        assert!(resolve.select_world(&[stuff], Some("foo")).is_ok());

        // Multiple packages, but still just one total world. Lookups
        // without a main package now fail.
        let empty = resolve.push_str(
            "./my-test.wit",
            r#"
                    package test:empty;
                "#,
        )?;
        assert!(resolve.select_world(&[stuff, empty], None).is_err());
        assert!(resolve.select_world(&[stuff, empty], Some("foo")).is_err());
        assert!(resolve.select_world(&[empty], None).is_err());
        assert!(resolve.select_world(&[empty], Some("foo")).is_err());

        Ok(())
    }

    /// Test selecting a world with multiple versions of a package name.
    #[test]
    fn select_world_versions() -> Result<()> {
        use wit_parser::Resolve;

        let mut resolve = Resolve::default();

        let _id = resolve.push_str(
            "./my-test.wit",
            r#"
                    package example:distraction;
                "#,
        )?;

        // When selecting with a version it's ok to drop the version when
        // there's only a single copy of that package in `Resolve`.
        let versions_1 = resolve.push_str(
            "./my-test.wit",
            r#"
                    package example:versions@1.0.0;

                    world foo { /* ... */ }
                "#,
        )?;
        assert!(resolve.select_world(&[versions_1], Some("foo")).is_ok());
        assert!(
            resolve
                .select_world(&[versions_1], Some("foo@1.0.0"))
                .is_err()
        );
        assert!(
            resolve
                .select_world(&[versions_1], Some("example:versions/foo"))
                .is_ok()
        );
        assert!(
            resolve
                .select_world(&[versions_1], Some("example:versions/foo@1.0.0"))
                .is_ok()
        );

        // However when a single package has multiple versions in a resolve
        // it's required to specify the version to select which one.
        let versions_2 = resolve.push_str(
            "./my-test.wit",
            r#"
                    package example:versions@2.0.0;

                    world foo { /* ... */ }
                "#,
        )?;
        assert!(
            resolve
                .select_world(&[versions_1, versions_2], Some("foo"))
                .is_err()
        );
        assert!(
            resolve
                .select_world(&[versions_1, versions_2], Some("foo@1.0.0"))
                .is_err()
        );
        assert!(
            resolve
                .select_world(&[versions_1, versions_2], Some("foo@2.0.0"))
                .is_err()
        );
        assert!(
            resolve
                .select_world(&[versions_1, versions_2], Some("example:versions/foo"))
                .is_err()
        );
        assert!(
            resolve
                .select_world(
                    &[versions_1, versions_2],
                    Some("example:versions/foo@1.0.0")
                )
                .is_ok()
        );
        assert!(
            resolve
                .select_world(
                    &[versions_1, versions_2],
                    Some("example:versions/foo@2.0.0")
                )
                .is_ok()
        );

        Ok(())
    }

    /// Test overriding a main package using name qualification
    #[test]
    fn select_world_override_qualification() -> Result<()> {
        use wit_parser::Resolve;

        let mut resolve = Resolve::default();

        let other = resolve.push_str(
            "./my-test.wit",
            r#"
                    package example:other;

                    world foo { }
                "#,
        )?;

        // A fully-qualified name overrides a main package.
        let fq = resolve.push_str(
            "./my-test.wit",
            r#"
                    package example:fq;

                    world bar { }
                "#,
        )?;
        assert!(resolve.select_world(&[other, fq], Some("foo")).is_err());
        assert!(resolve.select_world(&[other, fq], Some("bar")).is_err());
        assert!(
            resolve
                .select_world(&[other, fq], Some("example:other/foo"))
                .is_ok()
        );
        assert!(
            resolve
                .select_world(&[other, fq], Some("example:fq/bar"))
                .is_ok()
        );
        assert!(
            resolve
                .select_world(&[other, fq], Some("example:other/bar"))
                .is_err()
        );
        assert!(
            resolve
                .select_world(&[other, fq], Some("example:fq/foo"))
                .is_err()
        );

        Ok(())
    }

    /// Test selecting with fully-qualified world names.
    #[test]
    fn select_world_fully_qualified() -> Result<()> {
        use wit_parser::Resolve;

        let mut resolve = Resolve::default();

        let distraction = resolve.push_str(
            "./my-test.wit",
            r#"
                    package example:distraction;
                "#,
        )?;

        // If a package has multiple worlds, then we can't guess the world
        // even if we know the package.
        let multiworld = resolve.push_str(
            "./my-test.wit",
            r#"
                    package example:multiworld;

                    world foo { /* ... */ }

                    world bar { /* ... */ }
                "#,
        )?;
        assert!(
            resolve
                .select_world(&[distraction, multiworld], None)
                .is_err()
        );
        assert!(
            resolve
                .select_world(&[distraction, multiworld], Some("foo"))
                .is_err()
        );
        assert!(
            resolve
                .select_world(&[distraction, multiworld], Some("example:multiworld/foo"))
                .is_ok()
        );
        assert!(
            resolve
                .select_world(&[distraction, multiworld], Some("bar"))
                .is_err()
        );
        assert!(
            resolve
                .select_world(&[distraction, multiworld], Some("example:multiworld/bar"))
                .is_ok()
        );

        Ok(())
    }

    /// Test `select_world` with single and multiple packages.
    #[test]
    fn select_world_packages() -> Result<()> {
        use wit_parser::Resolve;

        let mut resolve = Resolve::default();

        // If there's a single package and only one world, that world is
        // the obvious choice.
        let wit1 = resolve.push_str(
            "./my-test.wit",
            r#"
                    package example:wit1;

                    world foo {
                        // ...
                    }
                "#,
        )?;
        assert!(resolve.select_world(&[wit1], None).is_ok());
        assert!(resolve.select_world(&[wit1], Some("foo")).is_ok());
        assert!(
            resolve
                .select_world(&[wit1], Some("example:wit1/foo"))
                .is_ok()
        );
        assert!(resolve.select_world(&[wit1], Some("bar")).is_err());
        assert!(
            resolve
                .select_world(&[wit1], Some("example:wit2/foo"))
                .is_err()
        );

        // If there are multiple packages, we need to be told which package
        // to use.
        let wit2 = resolve.push_str(
            "./my-test.wit",
            r#"
                    package example:wit2;

                    world foo { /* ... */ }
                "#,
        )?;
        assert!(resolve.select_world(&[wit1, wit2], None).is_err());
        assert!(resolve.select_world(&[wit1, wit2], Some("foo")).is_err());
        assert!(
            resolve
                .select_world(&[wit1, wit2], Some("example:wit1/foo"))
                .is_ok()
        );
        assert!(resolve.select_world(&[wit2], None).is_ok());
        assert!(resolve.select_world(&[wit2], Some("foo")).is_ok());
        assert!(
            resolve
                .select_world(&[wit2], Some("example:wit1/foo"))
                .is_ok()
        );
        assert!(resolve.select_world(&[wit1, wit2], Some("bar")).is_err());
        assert!(
            resolve
                .select_world(&[wit1, wit2], Some("example:wit2/foo"))
                .is_ok()
        );
        assert!(resolve.select_world(&[wit2], Some("bar")).is_err());
        assert!(
            resolve
                .select_world(&[wit2], Some("example:wit2/foo"))
                .is_ok()
        );

        Ok(())
    }

    #[test]
    fn span_preservation() -> Result<()> {
        let mut resolve = Resolve::default();
        let pkg = resolve.push_str(
            "test.wit",
            r#"
                package foo:bar;

                interface my-iface {
                    type my-type = u32;
                    my-func: func();
                }

                world my-world {
                    export my-export: func();
                }
            "#,
        )?;

        let iface_id = resolve.packages[pkg].interfaces["my-iface"];
        assert!(resolve.interfaces[iface_id].span.is_known());

        let type_id = resolve.interfaces[iface_id].types["my-type"];
        assert!(resolve.types[type_id].span.is_known());

        assert!(
            resolve.interfaces[iface_id].functions["my-func"]
                .span
                .is_known()
        );

        let world_id = resolve.packages[pkg].worlds["my-world"];
        assert!(resolve.worlds[world_id].span.is_known());

        let WorldItem::Function(f) =
            &resolve.worlds[world_id].exports[&WorldKey::Name("my-export".to_string())]
        else {
            panic!("expected function");
        };
        assert!(f.span.is_known());

        Ok(())
    }

    #[test]
    fn span_preservation_through_merge() -> Result<()> {
        let mut resolve1 = Resolve::default();
        resolve1.push_str(
            "test1.wit",
            r#"
                package foo:bar;

                interface iface1 {
                    type type1 = u32;
                    func1: func();
                }
            "#,
        )?;

        let mut resolve2 = Resolve::default();
        let pkg2 = resolve2.push_str(
            "test2.wit",
            r#"
                package foo:baz;

                interface iface2 {
                    type type2 = string;
                    func2: func();
                }
            "#,
        )?;

        let iface2_old_id = resolve2.packages[pkg2].interfaces["iface2"];
        let remap = resolve1.merge(resolve2)?;
        let iface2_id = remap.interfaces[iface2_old_id.index()].unwrap();

        assert!(resolve1.interfaces[iface2_id].span.is_known());

        let type2_id = resolve1.interfaces[iface2_id].types["type2"];
        assert!(resolve1.types[type2_id].span.is_known());

        assert!(
            resolve1.interfaces[iface2_id].functions["func2"]
                .span
                .is_known()
        );

        Ok(())
    }

    #[test]
    fn span_preservation_through_include() -> Result<()> {
        let mut resolve = Resolve::default();
        let pkg = resolve.push_str(
            "test.wit",
            r#"
                package foo:bar;

                world base {
                    export my-func: func();
                }

                world extended {
                    include base;
                }
            "#,
        )?;

        let base_id = resolve.packages[pkg].worlds["base"];
        let extended_id = resolve.packages[pkg].worlds["extended"];

        let WorldItem::Function(base_func) =
            &resolve.worlds[base_id].exports[&WorldKey::Name("my-func".to_string())]
        else {
            panic!("expected function");
        };
        assert!(base_func.span.is_known());

        let WorldItem::Function(extended_func) =
            &resolve.worlds[extended_id].exports[&WorldKey::Name("my-func".to_string())]
        else {
            panic!("expected function");
        };
        assert!(extended_func.span.is_known());

        Ok(())
    }

    #[test]
    fn span_preservation_through_include_with_rename() -> Result<()> {
        let mut resolve = Resolve::default();
        let pkg = resolve.push_str(
            "test.wit",
            r#"
                package foo:bar;

                world base {
                    export original-name: func();
                }

                world extended {
                    include base with { original-name as renamed-func }
                }
            "#,
        )?;

        let extended_id = resolve.packages[pkg].worlds["extended"];

        let WorldItem::Function(f) =
            &resolve.worlds[extended_id].exports[&WorldKey::Name("renamed-func".to_string())]
        else {
            panic!("expected function");
        };
        assert!(f.span.is_known());

        assert!(
            !resolve.worlds[extended_id]
                .exports
                .contains_key(&WorldKey::Name("original-name".to_string()))
        );

        Ok(())
    }

    /// Test that spans work when included world is defined after the including world
    #[test]
    fn span_preservation_through_include_reverse_order() -> Result<()> {
        let mut resolve = Resolve::default();
        let pkg = resolve.push_str(
            "test.wit",
            r#"
                package foo:bar;

                world extended {
                    include base;
                }

                world base {
                    export my-func: func();
                }
            "#,
        )?;

        let base_id = resolve.packages[pkg].worlds["base"];
        let extended_id = resolve.packages[pkg].worlds["extended"];

        let WorldItem::Function(base_func) =
            &resolve.worlds[base_id].exports[&WorldKey::Name("my-func".to_string())]
        else {
            panic!("expected function");
        };
        assert!(base_func.span.is_known());

        let WorldItem::Function(extended_func) =
            &resolve.worlds[extended_id].exports[&WorldKey::Name("my-func".to_string())]
        else {
            panic!("expected function");
        };
        assert!(extended_func.span.is_known());

        Ok(())
    }

    #[test]
    fn span_line_numbers() -> Result<()> {
        let mut resolve = Resolve::default();
        let pkg = resolve.push_source(
            "test.wit",
            "package foo:bar;

interface my-iface {
    type my-type = u32;
    my-func: func();
}

world my-world {
    export my-export: func();
}
",
        )?;

        let iface_id = resolve.packages[pkg].interfaces["my-iface"];
        let iface_span = resolve.interfaces[iface_id].span;
        let iface_loc = resolve.render_location(iface_span);
        assert!(
            iface_loc.contains(":3:"),
            "interface location was {iface_loc}"
        );

        let type_id = resolve.interfaces[iface_id].types["my-type"];
        let type_span = resolve.types[type_id].span;
        let type_loc = resolve.render_location(type_span);
        assert!(type_loc.contains(":4:"), "type location was {type_loc}");

        let func_span = resolve.interfaces[iface_id].functions["my-func"].span;
        let func_loc = resolve.render_location(func_span);
        assert!(func_loc.contains(":5:"), "function location was {func_loc}");

        let world_id = resolve.packages[pkg].worlds["my-world"];
        let world_span = resolve.worlds[world_id].span;
        let world_loc = resolve.render_location(world_span);
        assert!(world_loc.contains(":8:"), "world location was {world_loc}");

        let WorldItem::Function(export_func) =
            &resolve.worlds[world_id].exports[&WorldKey::Name("my-export".to_string())]
        else {
            panic!("expected function");
        };
        let export_loc = resolve.render_location(export_func.span);
        assert!(
            export_loc.contains(":9:"),
            "export location was {export_loc}"
        );

        Ok(())
    }

    #[test]
    fn span_line_numbers_through_merge() -> Result<()> {
        let mut resolve1 = Resolve::default();
        resolve1.push_source(
            "first.wit",
            "package foo:first;

interface iface1 {
    func1: func();
}
",
        )?;

        let mut resolve2 = Resolve::default();
        let pkg2 = resolve2.push_source(
            "second.wit",
            "package foo:second;

interface iface2 {
    func2: func();
}
",
        )?;

        let iface2_old_id = resolve2.packages[pkg2].interfaces["iface2"];
        let remap = resolve1.merge(resolve2)?;
        let iface2_id = remap.interfaces[iface2_old_id.index()].unwrap();

        let iface2_span = resolve1.interfaces[iface2_id].span;
        let iface2_loc = resolve1.render_location(iface2_span);
        assert!(
            iface2_loc.contains("second.wit"),
            "should reference second.wit, got {iface2_loc}"
        );
        assert!(
            iface2_loc.contains(":3:"),
            "interface should be on line 3, got {iface2_loc}"
        );

        let func2_span = resolve1.interfaces[iface2_id].functions["func2"].span;
        let func2_loc = resolve1.render_location(func2_span);
        assert!(
            func2_loc.contains("second.wit"),
            "should reference second.wit, got {func2_loc}"
        );
        assert!(
            func2_loc.contains(":4:"),
            "function should be on line 4, got {func2_loc}"
        );

        Ok(())
    }

    #[test]
    fn span_line_numbers_multiple_sources() -> Result<()> {
        let mut resolve = Resolve::default();

        let pkg1 = resolve.push_source(
            "first.wit",
            "package test:first;

interface first-iface {
    first-func: func();
}
",
        )?;

        let pkg2 = resolve.push_source(
            "second.wit",
            "package test:second;

interface second-iface {
    second-func: func();
}
",
        )?;

        let iface1_id = resolve.packages[pkg1].interfaces["first-iface"];
        let iface1_span = resolve.interfaces[iface1_id].span;
        let iface1_loc = resolve.render_location(iface1_span);
        assert!(
            iface1_loc.contains("first.wit"),
            "should reference first.wit, got {iface1_loc}"
        );
        assert!(
            iface1_loc.contains(":3:"),
            "interface should be on line 3, got {iface1_loc}"
        );

        let func1_span = resolve.interfaces[iface1_id].functions["first-func"].span;
        let func1_loc = resolve.render_location(func1_span);
        assert!(
            func1_loc.contains("first.wit"),
            "should reference first.wit, got {func1_loc}"
        );
        assert!(
            func1_loc.contains(":4:"),
            "function should be on line 4, got {func1_loc}"
        );

        let iface2_id = resolve.packages[pkg2].interfaces["second-iface"];
        let iface2_span = resolve.interfaces[iface2_id].span;
        let iface2_loc = resolve.render_location(iface2_span);
        assert!(
            iface2_loc.contains("second.wit"),
            "should reference second.wit, got {iface2_loc}"
        );
        assert!(
            iface2_loc.contains(":3:"),
            "interface should be on line 3, got {iface2_loc}"
        );

        let func2_span = resolve.interfaces[iface2_id].functions["second-func"].span;
        let func2_loc = resolve.render_location(func2_span);
        assert!(
            func2_loc.contains("second.wit"),
            "should reference second.wit, got {func2_loc}"
        );
        assert!(
            func2_loc.contains(":4:"),
            "function should be on line 4, got {func2_loc}"
        );

        Ok(())
    }

    #[test]
    fn span_preservation_for_fields_and_cases() -> Result<()> {
        use crate::TypeDefKind;

        let mut resolve = Resolve::default();
        let pkg = resolve.push_str(
            "test.wit",
            r#"
                package foo:bar;

                interface my-iface {
                    record my-record {
                        field1: u32,
                        field2: string,
                    }

                    flags my-flags {
                        flag1,
                        flag2,
                    }

                    variant my-variant {
                        case1,
                        case2(u32),
                    }

                    enum my-enum {
                        val1,
                        val2,
                    }
                }
            "#,
        )?;

        let iface_id = resolve.packages[pkg].interfaces["my-iface"];

        // Check record fields have spans
        let record_id = resolve.interfaces[iface_id].types["my-record"];
        let TypeDefKind::Record(record) = &resolve.types[record_id].kind else {
            panic!("expected record");
        };
        assert!(record.fields[0].span.is_known(), "field1 should have span");
        assert!(record.fields[1].span.is_known(), "field2 should have span");

        // Check flags have spans
        let flags_id = resolve.interfaces[iface_id].types["my-flags"];
        let TypeDefKind::Flags(flags) = &resolve.types[flags_id].kind else {
            panic!("expected flags");
        };
        assert!(flags.flags[0].span.is_known(), "flag1 should have span");
        assert!(flags.flags[1].span.is_known(), "flag2 should have span");

        // Check variant cases have spans
        let variant_id = resolve.interfaces[iface_id].types["my-variant"];
        let TypeDefKind::Variant(variant) = &resolve.types[variant_id].kind else {
            panic!("expected variant");
        };
        assert!(variant.cases[0].span.is_known(), "case1 should have span");
        assert!(variant.cases[1].span.is_known(), "case2 should have span");

        // Check enum cases have spans
        let enum_id = resolve.interfaces[iface_id].types["my-enum"];
        let TypeDefKind::Enum(e) = &resolve.types[enum_id].kind else {
            panic!("expected enum");
        };
        assert!(e.cases[0].span.is_known(), "val1 should have span");
        assert!(e.cases[1].span.is_known(), "val2 should have span");

        Ok(())
    }

    #[test]
    fn span_preservation_for_fields_through_merge() -> Result<()> {
        use crate::TypeDefKind;

        let mut resolve1 = Resolve::default();
        resolve1.push_str(
            "test1.wit",
            r#"
                package foo:bar;

                interface iface1 {
                    record rec1 {
                        f1: u32,
                    }
                }
            "#,
        )?;

        let mut resolve2 = Resolve::default();
        let pkg2 = resolve2.push_str(
            "test2.wit",
            r#"
                package foo:baz;

                interface iface2 {
                    record rec2 {
                        f2: string,
                    }

                    variant var2 {
                        c2,
                    }
                }
            "#,
        )?;

        let iface2_old_id = resolve2.packages[pkg2].interfaces["iface2"];
        let rec2_old_id = resolve2.interfaces[iface2_old_id].types["rec2"];
        let var2_old_id = resolve2.interfaces[iface2_old_id].types["var2"];

        let remap = resolve1.merge(resolve2)?;

        let rec2_id = remap.types[rec2_old_id.index()].unwrap();
        let TypeDefKind::Record(record) = &resolve1.types[rec2_id].kind else {
            panic!("expected record");
        };
        assert!(
            record.fields[0].span.is_known(),
            "field should have span after merge"
        );

        let var2_id = remap.types[var2_old_id.index()].unwrap();
        let TypeDefKind::Variant(variant) = &resolve1.types[var2_id].kind else {
            panic!("expected variant");
        };
        assert!(
            variant.cases[0].span.is_known(),
            "case should have span after merge"
        );

        Ok(())
    }

    #[test]
    fn param_spans_point_to_names() -> Result<()> {
        let source = "\
package foo:bar;

interface iface {
    my-func: func(a: u32, b: string);
}
";
        let mut resolve = Resolve::default();
        let pkg = resolve.push_str("test.wit", source)?;

        let iface_id = resolve.packages[pkg].interfaces["iface"];
        let func = &resolve.interfaces[iface_id].functions["my-func"];
        assert_eq!(func.params.len(), 2);
        for param in &func.params {
            let start = param.span.start() as usize;
            let end = param.span.end() as usize;
            let snippet = &source[start..end];
            assert_eq!(
                snippet, param.name,
                "param `{}` span points to {:?}",
                param.name, snippet
            );
        }

        Ok(())
    }

    #[test]
    fn push_groups_resolves_dep_before_main() -> Result<()> {
        // push_groups must topologically sort main + deps internally and succeed
        // even when the dep is listed after main in the caller's mental model.
        let dep = {
            let mut map = SourceMap::default();
            map.push_str(
                "file:///dep.wit",
                "package foo:dep;\ninterface i { type t = u32; }",
            );
            map.parse().map_err(|(_, e)| e)?
        };
        let main = {
            let mut map = SourceMap::default();
            map.push_str(
                "file:///main.wit",
                "package foo:main;\ninterface j { use foo:dep/i.{t}; type u = t; }",
            );
            map.parse().map_err(|(_, e)| e)?
        };
        let mut resolve = Resolve::default();
        resolve.push_groups(main, Vec::from([dep]))?;
        assert_eq!(resolve.packages.len(), 2);
        Ok(())
    }

    #[test]
    fn push_groups_cycle_error_contains_location() {
        // A cross-group cycle must produce an error message with a file URI and
        // line/col. This validates that source maps are merged into resolve.source_map
        // *before* toposort runs, so the span in the cycle error is resolvable.
        let a = {
            let mut map = SourceMap::default();
            map.push_str(
                "file:///a.wit",
                "package foo:a;\ninterface i { use foo:b/j.{}; }",
            );
            map.parse().unwrap()
        };
        let b = {
            let mut map = SourceMap::default();
            map.push_str(
                "file:///b.wit",
                "package foo:b;\ninterface j { use foo:a/i.{}; }",
            );
            map.parse().unwrap()
        };
        let mut resolve = Resolve::default();
        let err = resolve.push_groups(a, Vec::from([b])).unwrap_err();
        let msg = err.render(&resolve.source_map);
        assert!(
            msg.contains("file:///"),
            "cycle error should contain a file URI, got: {msg}"
        );
    }

    #[test]
    fn param_spans_preserved_through_merge() -> Result<()> {
        let mut resolve1 = Resolve::default();
        resolve1.push_str(
            "test1.wit",
            r#"
                package foo:bar;

                interface iface1 {
                    f1: func(x: u32);
                }
            "#,
        )?;

        let mut resolve2 = Resolve::default();
        let pkg2 = resolve2.push_str(
            "test2.wit",
            r#"
                package foo:baz;

                interface iface2 {
                    f2: func(y: string, z: bool);
                }
            "#,
        )?;

        let iface2_old_id = resolve2.packages[pkg2].interfaces["iface2"];

        let remap = resolve1.merge(resolve2)?;

        let iface2_id = remap.interfaces[iface2_old_id.index()].unwrap();
        let func = &resolve1.interfaces[iface2_id].functions["f2"];
        for param in &func.params {
            assert!(
                param.span.is_known(),
                "param `{}` should have span after merge",
                param.name
            );
        }

        Ok(())
    }

    /// Demonstrates the round-trip property: starting from a world with only
    /// exports, `importize` turns them into imports, then `exportize` turns
    /// them back. The resulting world has the same set of exports (by key)
    /// as the original.
    #[test]
    fn exportize_importize_roundtrip() -> Result<()> {
        let mut resolve = Resolve::default();
        let pkg = resolve.push_str(
            "test.wit",
            r#"
                package foo:bar;

                interface types {
                    type my-type = u32;
                }

                interface api {
                    use types.{my-type};
                    do-something: func(a: my-type) -> my-type;
                }

                world w {
                    export api;
                }
            "#,
        )?;
        let world_id = resolve.packages[pkg].worlds["w"];

        // Snapshot original export keys.
        let original_export_keys: Vec<String> = resolve.worlds[world_id]
            .exports
            .keys()
            .map(|k| resolve.name_world_key(k))
            .collect();
        assert!(!original_export_keys.is_empty());
        assert!(resolve.worlds[world_id].imports.iter().all(|(_, item)| {
            // Before importize the only imports should be elaborated
            // interface deps (all interface items).
            matches!(item, WorldItem::Interface { .. })
        }));

        // importize: exports -> imports, no exports remain.
        resolve.importize(world_id, Some("w-temp".to_string()))?;
        assert!(
            resolve.worlds[world_id].exports.is_empty(),
            "importize should leave no exports"
        );
        // The original exports should now appear as imports.
        for key in &original_export_keys {
            assert!(
                resolve.worlds[world_id]
                    .imports
                    .keys()
                    .any(|k| resolve.name_world_key(k) == *key),
                "expected `{key}` to be an import after importize"
            );
        }

        // exportize: imports -> exports, round-tripping back.
        resolve.exportize(world_id, Some("w-final".to_string()), None)?;
        assert!(
            !resolve.worlds[world_id].exports.is_empty(),
            "exportize should produce exports"
        );
        // The original export keys should be present as exports again.
        let final_export_keys: Vec<String> = resolve.worlds[world_id]
            .exports
            .keys()
            .map(|k| resolve.name_world_key(k))
            .collect();
        for key in &original_export_keys {
            assert!(
                final_export_keys.contains(key),
                "expected `{key}` to be an export after round-trip, got exports: {final_export_keys:?}"
            );
        }

        Ok(())
    }
}